1.2 Data Abstraction（算法 Algorithms 第4版）

1.2.1

package com.qiusongde;

import edu.princeton.cs.algs4.Point2D;
import edu.princeton.cs.algs4.StdDraw;
import edu.princeton.cs.algs4.StdOut;
import edu.princeton.cs.algs4.StdRandom;

public class Point2DClient {

    public static void main(String[] args) {

        int pointnumber;
        Point2D[] points;

        double closestdistance = 0;
        int closestpair1;
        int closestpair2;

        //usage
        if(args.length < 1)
        {
            StdOut.println("Usage: Point2DClient N");
            StdOut.println("Generate N random points");
            return;
        }

        pointnumber = Integer.parseInt(args[0]);
        points = new Point2D[pointnumber];

        //generate pointnumber random points
        StdDraw.setPenRadius(0.01);
        for(int i = 0; i < pointnumber; i++) {
            double x = StdRandom.random();
            double y = StdRandom.random();
            StdOut.printf("Point2D %d (%f %f)\n",i, x,y);
            points[i] = new Point2D(x, y);
            points[i].draw();
        }

        //find the closet pair of points
        closestdistance = points[0].distanceTo(points[1]);
        closestpair1 = 0;
        closestpair2 = 1;
        for(int i = 0; i< pointnumber; i++) {
            for(int j = i + 1; j < pointnumber; j++) {
                double distance = points[i].distanceTo(points[j]);
                StdOut.printf("distance from %d to %d is %f\n", i,j,distance);
                if(distance < closestdistance) {
                    closestdistance = distance;
                    closestpair1 = i;
                    closestpair2 = j;
                }
            }
        }

        StdOut.printf("closest distance is %f from %d to %d\n", closestdistance, closestpair1, closestpair2);
        points[closestpair1].drawTo(points[closestpair2]);

    }

}

Point2D 0 (0.757256 0.133583)
Point2D 1 (0.276017 0.882527)
Point2D 2 (0.724763 0.603589)
Point2D 3 (0.186021 0.002862)
Point2D 4 (0.768848 0.125174)
Point2D 5 (0.434056 0.732299)
Point2D 6 (0.119133 0.335418)
Point2D 7 (0.177490 0.382443)
Point2D 8 (0.726221 0.595894)
Point2D 9 (0.953359 0.845522)
distance from 0 to 1 is 0.890230
distance from 0 to 2 is 0.471128
distance from 0 to 3 is 0.586001
distance from 0 to 4 is 0.014321
distance from 0 to 5 is 0.680382
distance from 0 to 6 is 0.669282
distance from 0 to 7 is 0.630920
distance from 0 to 8 is 0.463352
distance from 0 to 9 is 0.738453
distance from 1 to 2 is 0.528375
distance from 1 to 3 is 0.884257
distance from 1 to 4 is 0.903585
distance from 1 to 5 is 0.218048
distance from 1 to 6 is 0.569158
distance from 1 to 7 is 0.509697
distance from 1 to 8 is 0.533706
distance from 1 to 9 is 0.678352
distance from 2 to 3 is 0.806918
distance from 2 to 4 is 0.480442
distance from 2 to 5 is 0.317926
distance from 2 to 6 is 0.662347
distance from 2 to 7 is 0.590265
distance from 2 to 8 is 0.007831
distance from 2 to 9 is 0.332847
distance from 3 to 4 is 0.595523
distance from 3 to 5 is 0.770454
distance from 3 to 6 is 0.339217
distance from 3 to 7 is 0.379678
distance from 3 to 8 is 0.802187
distance from 3 to 9 is 1.139685
distance from 4 to 5 is 0.693316
distance from 4 to 6 is 0.682885
distance from 4 to 7 is 0.644896
distance from 4 to 8 is 0.472647
distance from 4 to 9 is 0.743603
distance from 5 to 6 is 0.506647
distance from 5 to 7 is 0.433849
distance from 5 to 8 is 0.322439
distance from 5 to 9 is 0.531503
distance from 6 to 7 is 0.074946
distance from 6 to 8 is 0.660609
distance from 6 to 9 is 0.977823
distance from 7 to 8 is 0.588784
distance from 7 to 9 is 0.903556
distance from 8 to 9 is 0.337499
closest distance is 0.007831 from 2 to 8

1.2.2和1.2.3解法跟1.2.1类似，略

1.2.4

输出结果为：

world

hello

因为最后string1是“world”的引用

string2是“hello”的引用

package com.qiusongde;

import edu.princeton.cs.algs4.StdOut;

public class Exercise124 {

    public static void main(String[] args) {
        String string1 = "hell0";
        String string2 = string1;
        string1 = "world";
        StdOut.println(string1);
        StdOut.println(string2);
    }

}

结果：

1.2.5

package com.qiusongde;

import edu.princeton.cs.algs4.StdOut;

public class Exercise125 {

    public static void main(String[] args) {
        String s = "Hello World";
        s.toUpperCase();
        s.substring(6, 11);
        StdOut.println(s);
    }

}

程序从头到尾s指向的对象都没有改变过，所以最后输出“Hello World”

1.2.6

package com.qiusongde;

import edu.princeton.cs.algs4.StdOut;

public class Exercise126 {

    public static void main(String[] args) {

        String s = "ACTGACG";
        String t = "ACGACTG";

        StdOut.println(s);
        StdOut.println(t);

        if(isCircularRotation(s,t))
            StdOut.println("String s and String t is Circular Rotation");
        else
            StdOut.println("String s and String t isn't Circular Rotation");
    }

    public static boolean isCircularRotation(String s, String t) {

        if(s.length() == t.length() && s.concat(s).indexOf(t) >= 0)
            return true;
        return false;

    }

}

1.2.7

起到反转String的作用，以下是程序测试。

package com.qiusongde;

import edu.princeton.cs.algs4.StdOut;

public class Exercise127 {

    public static void main(String[] args) {
        String test = "I am a student!";
        StdOut.println(test);
        StdOut.println(mystery(test));
    }

    public static String mystery(String s) {
        int N = s.length();
        if(N <= 1) return s;
        String a = s.substring(0, N/2);
        String b = s.substring(N/2, N);
        return mystery(b) + mystery(a);
    }

}

1.2.8

书中有答案

1.2.9

package com.qiusongde;

import java.util.Arrays;

import edu.princeton.cs.algs4.Counter;
import edu.princeton.cs.algs4.In;
import edu.princeton.cs.algs4.StdIn;
import edu.princeton.cs.algs4.StdOut;

public class Exercise129 {

    public static int rank(int key, int[] a, Counter counter) {

        counter.increment();

        //array must be sorted
        int lo = 0;
        int hi = a.length - 1;

        while(lo <= hi) {

            int mid = lo + (hi - lo) / 2;

            if(key < a[mid])
                hi = mid -1;
            else if(key > a[mid])
                lo = mid + 1;
            else
                return mid;

        }

        return -1;
    }

    public static void main(String[] args) {

        Counter counter = new Counter("rank");
        int[] whitelist = In.readInts(args[0]);

        Arrays.sort(whitelist);
        StdOut.println(Arrays.toString(whitelist));

        while(!StdIn.isEmpty()) {
            int key = StdIn.readInt();
            if(rank(key, whitelist, counter) < 0)
                StdOut.println(key);
        }

        StdOut.println(counter);
    }
}

1.2.10

package com.qiusongde;

import edu.princeton.cs.algs4.StdDraw;
import edu.princeton.cs.algs4.StdIn;
import edu.princeton.cs.algs4.StdOut;
import edu.princeton.cs.algs4.StdRandom;

public class VisualCounter {

    private int maxx;
    private int maxabsy;

    private int count = 0;
    private int times = 0;

    public VisualCounter(int N, int max) {
        StdDraw.setXscale(0, N);
        StdDraw.setYscale(-max, max);
        StdDraw.setPenRadius(0.005);
        StdDraw.line(0, 0, N, 0);
        maxx = N;
        maxabsy = max;
    }

    public void increment() {
        times++;
        count++;
        StdDraw.point(times, count);
    }

    public void decrement() {
        times++;
        count--;
        StdDraw.point(times, count);
    }

    public static void main(String[] args) {

        VisualCounter counter = new VisualCounter(100, 100);

        for(int i = 0; i < 100; i++) {
            if(StdRandom.bernoulli()) {
                counter.increment();
            }
            else {
                counter.decrement();
            }
        }

    }

}

1.2.11

package com.qiusongde;

public class SmartDate {

    private final int month;
    private final int day;
    private final int year;

    private final int[] MAXDAY = {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

    public SmartDate(int m, int d, int y) {
        if(!isValid(m, d, y))
            throw new IllegalArgumentException("Invalid Date:" + m + "/" + d + "/" + y);
        month = m;
        day = d;
        year = y;
    }

    private boolean isValid(int m, int d, int y) {
        if(m < 1 || m > 12)
            return false;
        if(d < 1 || d > MAXDAY[m-1])
            return false;
        if(m == 2 && d == 29 && !isLeapYear(y))
            return false;
        return true;
    }

    private boolean isLeapYear(int y) {
        if(y % 4 == 0 && y % 100 != 0 || y % 400 == 0)
            return true;
        return false;
    }

    public int month() {
        return month;
    }

    public int day() {
        return day;
    }

    public int year() {
        return year;
    }

    public String toString() {
        return month() +"/" + day() + "/" + year();
    }

    public static void main(String[] args) {

        try {
            SmartDate date1 = new SmartDate(13, 31, 2009);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date2 = new SmartDate(6, 32, 2009);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date3 = new SmartDate(2, 29, 2017);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date4 = new SmartDate(2, 29, 2016);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date5 = new SmartDate(2, 29, 2000);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date6 = new SmartDate(2, 29, 2100);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date7 = new SmartDate(6, 31, 2009);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date8 = new SmartDate(2, 25, 2009);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

    }

}

java.lang.IllegalArgumentException: Invalid Date:13/31/2009
    at com.qiusongde.SmartDate.<init>(SmartDate.java:13)
    at com.qiusongde.SmartDate.main(SmartDate.java:54)
java.lang.IllegalArgumentException: Invalid Date:6/32/2009
    at com.qiusongde.SmartDate.<init>(SmartDate.java:13)
    at com.qiusongde.SmartDate.main(SmartDate.java:60)
java.lang.IllegalArgumentException: Invalid Date:2/29/2017
    at com.qiusongde.SmartDate.<init>(SmartDate.java:13)
    at com.qiusongde.SmartDate.main(SmartDate.java:66)
java.lang.IllegalArgumentException: Invalid Date:2/29/2100
    at com.qiusongde.SmartDate.<init>(SmartDate.java:13)
    at com.qiusongde.SmartDate.main(SmartDate.java:84)
java.lang.IllegalArgumentException: Invalid Date:6/31/2009
    at com.qiusongde.SmartDate.<init>(SmartDate.java:13)
    at com.qiusongde.SmartDate.main(SmartDate.java:90)

1.2.12

　　　　　public String dayOfTheWeek() {
        //assume that the date is in the 21st century
        //in 1/1/2000 is Saturday

        int offsetday = 0;//the offset of day from 1/1/2000
        final String[] WEEKDAY = {"Saturday", "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday"};

        //year
        offsetday += 365 * (year - 2000);//ignore leap year first

        //month
        for(int i = 0; i < month - 1; i++) {
            offsetday += MAXDAY[i];//add according to the max day
        }
        if(month > 2) {
            if(!isLeapYear(year))
                offsetday--;
        }

        //day
        offsetday += day - 1;

        //add leapyear day in February
        offsetday += Leapyears(year);

        return WEEKDAY[offsetday % 7];
    }

    //return the number of leap year since 2000
    private int Leapyears(int year) {

        if(year == 2000)
            return 0;

        int number = 0;
        int offset = year - 2001;

        number += offset/4;//how many 4 years since 2001
        number -= offset/100 ;//how many 100 years since 2001
        number += offset/400;//how many 400 years since 2001
        number++;//2000 year is leap year

        return number;
    }

        SmartDate date1 = new SmartDate(2, 25, 2017);
        StdOut.println(date1.dayOfTheWeek());

        SmartDate date2 = new SmartDate(1, 1, 2000);
        StdOut.println(date2.dayOfTheWeek());

        SmartDate date3 = new SmartDate(1, 1, 2100);
        StdOut.println(date3.dayOfTheWeek());

        SmartDate date4 = new SmartDate(1, 1, 2101);
        StdOut.println(date4.dayOfTheWeek());

        SmartDate date5 = new SmartDate(1, 1, 2400);
        StdOut.println(date5.dayOfTheWeek());

        SmartDate date6 = new SmartDate(1, 1, 2401);
        StdOut.println(date6.dayOfTheWeek());            

1.2.13

package com.qiusongde;

import edu.princeton.cs.algs4.Date;
import edu.princeton.cs.algs4.StdOut;

public class Transaction {

    private String name;
    private Date date;
    private double amount;

    public Transaction(String who, Date when, double amount) {
        //should check the argument
        name = who;
        date = when;
        this.amount = amount;
    }

    public String who() {
        return name;
    }

    public Date when() {
        return date;
    }

    public double amount() {
        return amount;
    }

    public String toString() {
        return date + " " + name + " " + amount;
    }

    public static void main(String[] args) {
        Date date = new Date(2, 26, 2017);
        Transaction transaction = new Transaction("Songde Qiu", date, 500);
        StdOut.println(transaction);
    }

}

1.2.14

package com.qiusongde;

import edu.princeton.cs.algs4.Date;
import edu.princeton.cs.algs4.StdOut;

public class Transaction {

    private String name;
    private Date date;
    private double amount;

    public Transaction(String who, Date when, double amount) {
        //should check the argument
        name = who;
        date = when;
        this.amount = amount;
    }

    public String who() {
        return name;
    }

    public Date when() {
        return date;
    }

    public double amount() {
        return amount;
    }

    public String toString() {
        return date + " " + name + " " + amount;
    }

    public boolean equals(Object x) {

        if(this == x)
            return true;
        if(x == null)
            return false;
        if(this.getClass() != x.getClass())
            return false;

        Transaction that = (Transaction) x;
        if(!name.equals(that.name))
            return false;
        if(!date.equals(that.date))
            return false;
        if(amount != that.amount)
            return false;

        return true;
    }

    public static void main(String[] args) {

        Date date = new Date(2, 26, 2017);
        Transaction transaction = new Transaction("Songde Qiu", date, 500);

        Date date1 = new Date(2, 27, 2017);
        Transaction transaction1 = new Transaction("Songde Qiu", date1, 500);

        Transaction transaction2 = new Transaction("Songde Qiu", date, 500);

        StdOut.println(transaction.equals(transaction));
        StdOut.println(transaction.equals(null));
        StdOut.println(transaction.equals(date));
        StdOut.println(transaction.equals(transaction1));
        StdOut.println(transaction.equals(transaction2));

    }

}

1.2.15

书中已经由答案

    public static int[] readInts(String name) {
        In in = new In(name);

        String input = in.readAll();
        String[] words = input.split("\\s+");

        int[] ints = new int[words.length];
        for(int i = 0; i < words.length; i++) {
            ints[i] = Integer.parseInt(words[i]);
        }

        return ints;
    }

其中"\\s+" \\s表示空格、回车、换行符等空白符，+表示多个的意思。

1.2.16

package com.qiusongde;

import edu.princeton.cs.algs4.StdOut;

public class Rational {

    private final long numerator;
    private final long denominator;

    public Rational(long numerator, long denominator) {

        if(denominator == 0)
            throw new ArithmeticException("denominator is zero");

        //find the greatest common divisor
        //to reduce the result to the simplest fraction
        long gcd = gcd(numerator, denominator);

        //assure denominator is positive
        if(denominator < 0) {
            this.numerator = -numerator/gcd;
            this.denominator = -denominator/gcd;
        } else {
            this.numerator = numerator/gcd;
            this.denominator = denominator/gcd;
        }

    }

    //the greatest commom divisor of |p| and |q|
    private long gcd(long p, long q) {

        if(p < 0)
            p = -p;

        if(q < 0)
            q = -q;

        if(q == 0)
            return p;

        long r = p % q;
        return gcd(q, r);
    }

    public Rational plus(Rational b) {
        return new Rational((this.numerator * b.denominator + this.denominator * b.numerator), this.denominator * b.denominator);
    }

    public Rational minus(Rational b) {
        return this.plus(new Rational(-b.numerator, b.denominator));
    }

    public Rational times(Rational b) {
        return new Rational(this.numerator * b.numerator, this.denominator * b.denominator);
    }

    public Rational divides(Rational b) {
        return new Rational(this.numerator * b.denominator, this.denominator * b.numerator);
    }

    public boolean equals(Object that) {

        if(this == that)
            return true;

        if(that == null)
            return false;

        if(this.getClass() != that.getClass())
            return false;

        Rational other = (Rational) that;
        return (this.numerator == other.numerator) && (this.denominator == other.denominator);
    }

    public String toString() {
        return numerator + "/" + denominator;
    }

    public static void main(String[] args) {

        Rational a = new Rational(3, 9);
        Rational b = new Rational(-2, 8);
        Rational c = new Rational(1, -7);
        Rational d = new Rational(-1, -3);

        StdOut.println("a Rational(3, 9):" + a.toString());
        StdOut.println("b Rational(-2, 8):" + b.toString());
        StdOut.println("c Rational(1, -7):" + c.toString());
        StdOut.println("d Rational(-1, -3):" + d.toString());

        StdOut.println("a + d:" + a.plus(d).toString());
        StdOut.println("a - b:" + a.minus(b).toString());
        StdOut.println("b * c:" + b.times(c).toString());
        StdOut.println("c / d:" + c.divides(d).toString());
        StdOut.println("a == d?" + a.equals(d));
        StdOut.println("b == c?" + b.equals(c));

        new Rational(1, 0);
    }

}

1.2.17

//1.2.16
//1.2.17
package com.qiusongde;

import edu.princeton.cs.algs4.StdOut;

public class Rational {

    private final long numerator;
    private final long denominator;

    private final long MAX = Long.MAX_VALUE;
    private final long MIN = Long.MIN_VALUE;

    public Rational(long numerator, long denominator) {

        if(denominator == 0)
            throw new ArithmeticException("denominator is zero");

        //find the greatest common divisor
        //to reduce the result to the simplest fraction
        long gcd = gcd(numerator, denominator);

        //assure denominator is positive
        if(denominator < 0) {
            this.numerator = -numerator/gcd;
            this.denominator = -denominator/gcd;
        } else {
            this.numerator = numerator/gcd;
            this.denominator = denominator/gcd;
        }

    }

    //the greatest commom divisor of |p| and |q|
    private long gcd(long p, long q) {

        if(p < 0)
            p = -p;

        if(q < 0)
            q = -q;

        if(q == 0)
            return p;

        long r = p % q;
        return gcd(q, r);
    }

    //true --> will overflow
    //false --> will not overflow
    private boolean is_overflow_plus(long a, long b) {
        return a >= 0 ? MAX - a < b : MIN - a > b;
    }

    //true --> will overflow
    //false --> will not overflow
    private boolean is_overflow_times(long a, long b) {

        if(a == 0 || b == 0) {
            return false;
        }

        if( a >= 0 && b >=0  ) {
            return MAX / a < b;
        }
        else if( a < 0 && b < 0 ) {
            return MAX / a > b;
        }
        else {
            return a < 0 ?  MIN / a < b: MIN / a > b ;
        }

    }

    public Rational plus(Rational b) {

        assert is_overflow_times(this.numerator, b.denominator) : "plus overflows";
        assert is_overflow_times(this.denominator, b.numerator) : "plus overflows";
        assert is_overflow_plus(this.numerator * b.denominator, this.denominator * b.numerator) : "plus overflows";

        return new Rational((this.numerator * b.denominator + this.denominator * b.numerator), this.denominator * b.denominator);
    }

    public Rational minus(Rational b) {
        return this.plus(new Rational(-b.numerator, b.denominator));
    }

    public Rational times(Rational b) {

        assert is_overflow_times(this.numerator, b.numerator) : "times overflows";
        assert is_overflow_times(this.denominator, b.denominator) : "times overflows";

        return new Rational(this.numerator * b.numerator, this.denominator * b.denominator);
    }

    public Rational divides(Rational b) {
        return this.times(new Rational(b.denominator, b.numerator));
    }

    public boolean equals(Object that) {

        if(this == that)
            return true;

        if(that == null)
            return false;

        if(this.getClass() != that.getClass())
            return false;

        Rational other = (Rational) that;
        return (this.numerator == other.numerator) && (this.denominator == other.denominator);
    }

    public String toString() {
        return numerator + "/" + denominator;
    }

    public static void main(String[] args) {

//        Rational a = new Rational(3, 9);
//        Rational b = new Rational(-2, 8);
//        Rational c = new Rational(1, -7);
//        Rational d = new Rational(-1, -3);
//
//        StdOut.println("a Rational(3, 9):" + a.toString());
//        StdOut.println("b Rational(-2, 8):" + b.toString());
//        StdOut.println("c Rational(1, -7):" + c.toString());
//        StdOut.println("d Rational(-1, -3):" + d.toString());
//
//        StdOut.println("a + d:" + a.plus(d).toString());
//        StdOut.println("a - b:" + a.minus(b).toString());
//        StdOut.println("b * c:" + b.times(c).toString());
//        StdOut.println("c / d:" + c.divides(d).toString());
//        StdOut.println("a == d?" + a.equals(d));
//        StdOut.println("b == c?" + b.equals(c));
//
//        new Rational(1, 0);

        Rational a = new Rational(Long.MAX_VALUE, 2);
        Rational b = new Rational(2, Long.MAX_VALUE);
        StdOut.println(a.is_overflow_plus(Long.MAX_VALUE - 1, 1));
        StdOut.println(a.is_overflow_plus(Long.MAX_VALUE - 1, 2));

        StdOut.println(a.is_overflow_plus(Long.MIN_VALUE + 2, -2));
        StdOut.println(a.is_overflow_plus(Long.MIN_VALUE + 2, -3));

        StdOut.println(a.is_overflow_times(Long.MAX_VALUE/2, 3));
        StdOut.println(a.is_overflow_times(Long.MAX_VALUE/2, 2));

        StdOut.println(a.is_overflow_times(-(Long.MAX_VALUE/2), -3));
        StdOut.println(a.is_overflow_times(-(Long.MAX_VALUE/2), -2));

        StdOut.println(a.is_overflow_times(-(Long.MIN_VALUE/2), -3));
        StdOut.println(a.is_overflow_times(-(Long.MIN_VALUE/2), -2));

        StdOut.println(a.is_overflow_times((Long.MIN_VALUE/2), 3));
        StdOut.println(a.is_overflow_times((Long.MIN_VALUE/2), 2));

        StdOut.println(a.plus(b));
        StdOut.println(a.times(b));
    }

}

开启assert之前的运行结果：

开始之后的运行结果：

1.2.18

https://www.johndcook.com/blog/standard_deviation/

Accurately computing running variance

1.2.19

//1.2.11
//1.2.12
//1.2.19
package com.qiusongde;

import edu.princeton.cs.algs4.StdOut;

public class SmartDate {

    private final int month;
    private final int day;
    private final int year;

    private final int[] MAXDAY = {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

    public SmartDate(String date) {

        String[] fields = date.split("/");

        if(fields.length < 3) {
            throw new IllegalArgumentException("should more than 3 arguments");
        }

        int m = Integer.parseInt(fields[0]);
        int d = Integer.parseInt(fields[1]);
        int y = Integer.parseInt(fields[2]);

        if(!isValid(m, d, y))
            throw new IllegalArgumentException("Invalid Date:" + m + "/" + d + "/" + y);

        month = m;
        day = d;
        year = y;
    }

    public SmartDate(int m, int d, int y) {
        if(!isValid(m, d, y))
            throw new IllegalArgumentException("Invalid Date:" + m + "/" + d + "/" + y);
        month = m;
        day = d;
        year = y;
    }

    private boolean isValid(int m, int d, int y) {
        if(m < 1 || m > 12)
            return false;
        if(d < 1 || d > MAXDAY[m-1])
            return false;
        if(m == 2 && d == 29 && !isLeapYear(y))
            return false;
        return true;
    }

    private boolean isLeapYear(int y) {
        if(y % 4 == 0 && y % 100 != 0 || y % 400 == 0)
            return true;
        return false;
    }

    public int month() {
        return month;
    }

    public int day() {
        return day;
    }

    public int year() {
        return year;
    }

    public String dayOfTheWeek() {
        //assume that the date is in the 21st century
        //in 1/1/2000 is Saturday

        int offsetday = 0;//the offset of day from 1/1/2000
        final String[] WEEKDAY = {"Saturday", "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday"};

        //year
        offsetday += 365 * (year - 2000);//ignore leap year first

        //month
        for(int i = 0; i < month - 1; i++) {
            offsetday += MAXDAY[i];//add according to the max day
        }
        if(month > 2) {
            if(!isLeapYear(year))
                offsetday--;
        }

        //day
        offsetday += day - 1;

        //add leapyear day in February
        offsetday += Leapyears(year);

        return WEEKDAY[offsetday % 7];
    }

    //return the number of leap year since 2000
    private int Leapyears(int year) {

        if(year == 2000)
            return 0;

        int number = 0;
        int offset = year - 2001;

        number += offset/4;//how many 4 years since 2001
        number -= offset/100 ;//how many 100 years since 2001
        number += offset/400;//how many 400 years since 2001
        number++;//2000 year is leap year

        return number;
    }

    public String toString() {
        return month +"/" + day + "/" + year;
    }

    public static void main(String[] args) {

//        try {
//            SmartDate date1 = new SmartDate(13, 31, 2009);
//        } catch (IllegalArgumentException e) {
//            e.printStackTrace();
//        }
//
//        try {
//            SmartDate date2 = new SmartDate(6, 32, 2009);
//        } catch (IllegalArgumentException e) {
//            e.printStackTrace();
//        }
//
//        try {
//            SmartDate date3 = new SmartDate(2, 29, 2017);
//        } catch (IllegalArgumentException e) {
//            e.printStackTrace();
//        }
//
//        try {
//            SmartDate date4 = new SmartDate(2, 29, 2016);
//        } catch (IllegalArgumentException e) {
//            e.printStackTrace();
//        }
//
//        try {
//            SmartDate date5 = new SmartDate(2, 29, 2000);
//        } catch (IllegalArgumentException e) {
//            e.printStackTrace();
//        }
//
//        try {
//            SmartDate date6 = new SmartDate(2, 29, 2100);
//        } catch (IllegalArgumentException e) {
//            e.printStackTrace();
//        }
//
//        try {
//            SmartDate date7 = new SmartDate(6, 31, 2009);
//        } catch (IllegalArgumentException e) {
//            e.printStackTrace();
//        }
//
//        try {
//            SmartDate date8 = new SmartDate(2, 25, 2009);
//        } catch (IllegalArgumentException e) {
//            e.printStackTrace();
//        }

//        SmartDate date1 = new SmartDate(2, 25, 2017);
//        StdOut.println(date1.dayOfTheWeek());
//
//        SmartDate date2 = new SmartDate(1, 1, 2000);
//        StdOut.println(date2.dayOfTheWeek());
//
//        SmartDate date3 = new SmartDate(1, 1, 2100);
//        StdOut.println(date3.dayOfTheWeek());
//
//        SmartDate date4 = new SmartDate(1, 1, 2101);
//        StdOut.println(date4.dayOfTheWeek());
//
//
//        SmartDate date5 = new SmartDate(1, 1, 2400);
//        StdOut.println(date5.dayOfTheWeek());
//
//        SmartDate date6 = new SmartDate(1, 1, 2401);
//        StdOut.println(date6.dayOfTheWeek());

        try {
            SmartDate date1 = new SmartDate("13/31/2009");
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date2 = new SmartDate("6/32/2009");
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date3 = new SmartDate("2/29/2017");
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date4 = new SmartDate("2/29/2016");
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date5 = new SmartDate("2/29/2000");
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date6 = new SmartDate("2/29/2100");
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date7 = new SmartDate("6/31/2009");
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date8 = new SmartDate("2/25/2009");
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

        try {
            SmartDate date9 = new SmartDate("2/25");
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }

    }

}

//1.2.13
//1.2.14
//1.2.19
package com.qiusongde;

import edu.princeton.cs.algs4.Date;
import edu.princeton.cs.algs4.StdOut;

public class Transaction {

    private final String name;
    private final Date date;
    private final double amount;

    public Transaction(String transaction) {

        String[] fields = transaction.split("\\s+");

        if(fields.length < 3) {
            throw new IllegalArgumentException("should more than 3 arguments");
        }

        name = fields[0];
        date = new Date(fields[1]);
        amount = Double.parseDouble(fields[2]);

    }

    public Transaction(String who, Date when, double amount) {
        //should check the argument
        name = who;
        date = when;
        this.amount = amount;
    }

    public String who() {
        return name;
    }

    public Date when() {
        return date;
    }

    public double amount() {
        return amount;
    }

    public String toString() {
        return date + " " + name + " " + amount;
    }

    public boolean equals(Object x) {

        if(this == x)
            return true;
        if(x == null)
            return false;
        if(this.getClass() != x.getClass())
            return false;

        Transaction that = (Transaction) x;
        return (this.amount == that.amount) && (this.name.equals(that.name))
                && (this.date.equals(that.date));
    }

    public static void main(String[] args) {

//        Date date = new Date(2, 26, 2017);
//        Transaction transaction = new Transaction("Songde Qiu", date, 500);
//
//        Date date1 = new Date(2, 27, 2017);
//        Transaction transaction1 = new Transaction("Songde Qiu", date1, 500);
//
//        Transaction transaction2 = new Transaction("Songde Qiu", date, 500);
//
//        StdOut.println(transaction.equals(transaction));
//        StdOut.println(transaction.equals(null));
//        StdOut.println(transaction.equals(date));
//        StdOut.println(transaction.equals(transaction1));
//        StdOut.println(transaction.equals(transaction2));

        Transaction transaction1 = new Transaction("Songde    2/26/2017    500");
        StdOut.println(transaction1);
        Transaction transaction2 = new Transaction("Songde    2/27/2017    500");
        StdOut.println(transaction2);

        try {
            Transaction transaction3 = new Transaction("Songde    2/27/2017");
            StdOut.println(transaction3);
        } catch(IllegalArgumentException e) {
            e.printStackTrace();
        }

    }

}