java高精度，大数

package 高精度幂;

import java.math.BigDecimal;
import java.util.Scanner;

public class Main{
    public static void main(String[] args) {
        Scanner cin = new Scanner(System.in);
        while (cin.hasNext()) {
            BigDecimal bd = new BigDecimal(cin.next());
            BigDecimal result = bd.pow(cin.nextInt());
            String s = result.stripTrailingZeros().toPlainString(); //去掉尾零
            if(s.startsWith("0"))        //去掉首零
                s=s.substring(1);
            System.out.println(s);
        }
    }

}

2.HDU1042 N!

import java.math.*;
import java.util.*;

public class Main {
    public static BigInteger num;

    public static void main(String args[]) {
        Scanner cin = new Scanner(System.in);

        while (cin.hasNext()) {
            num = BigInteger.ONE;
            int n = cin.nextInt();
            num = BigInteger.ONE;
            for (int i = 1; i <= n; i++) {
                num= num.multiply(BigInteger.valueOf(i));
            }
            System.out.println(num);
        }
    }
}

4.HDU1753 加法

import java.math.BigDecimal;
import java.util.Scanner;
import java.util.*;

public class Main {

    public static void main(String[] args) {
        Scanner cin = new Scanner(System.in);
        while(cin.hasNext()){
            BigDecimal a=cin.nextBigDecimal();
            BigDecimal b=cin.nextBigDecimal();
            String s = a.add(b).stripTrailingZeros().toPlainString();
            System.out.println(s);
        }

    }

}

比较函数

 if(a.compareTo(b)<)//此时由于a<b，则返回值小于0；
        {
            System.out.println("a<b");
        }
        if(b.equals(c))//此时由于b==c,则返回值等于0；
        {
            System.out.println("b==c");
        }
        if(d.compareTo(c)>)//此时由于d>c，则返回值大于0；
        {
            System.out.println("d>c");
        }

(4) 
对于输出浮点数保留几位小数的问题，可以使用DecimalFormat类，

import java.text.*; 
DecimalFormat f = new DecimalFormat("#.00#"); 
DecimalFormat g = new DecimalFormat("0.000"); 
double a = 123.45678, b = 0.12; 
System.out.println(f.format(a)); 
System.out.println(f.format(b)); 
System.out.println(g.format(b));

这里0指一位数字，#指除0以外的数字。

string中的字符串不可变，stringbuild或者stringbuffer中的可变，还有insert与append方法

reverse函数

package test1;

public class TestReverse {
public static void main(String[] args) {
String str2 = "Hello";
str2 = new StringBuffer(str2).reverse().toString();
System.out.println(str2);

String message = "Hello";
StringBuilder rev = new StringBuilder();
for (int i = message.length() - 1; i >= 0; i--)
rev.append(message.charAt(i));
System.out.println(rev.toString());

}

}

java中字符串String是不可以修改的，要修改只能转换为字符数组.

例程：

1. import java.io.*;
2. import java.math.*;
3. import java.util.*;
4. import java.text.*;
5. public class Main
6. {
7. public static void main(String[] args)
8. {
9. int i;
10. Scanner cin = new Scanner (new BufferedInputStream(System.in));
11. String st = "abcdefg";
12. System.out.println(st.charAt(0)); // st.charAt(i)就相当于st[i].
13. char [] ch;
14. ch = st.toCharArray(); // 字符串转换为字符数组.
15. for (i = 0; i < ch.length; i++) ch[i] += 1;
16. System.out.println(ch); // 输入为“bcdefgh”.
17. if (st.startsWith("a")) // 如果字符串以'0'开头.
18. {
19. st = st.substring(1); // 则从第1位开始copy(开头为第0位).
20. }
21. }
22. }

函数：Arrays.sort();

例程：

1. import java.io.*;
2. import java.math.*;
3. import java.util.*;
4. import java.text.*;
5. public class Main
6. {
7. public static void main(String[] args)
8. {
9. Scanner cin = new Scanner (new BufferedInputStream(System.in));
10. int n = cin.nextInt();
11. int a[] = new int [n];
12. for (int i = 0; i < n; i++) a[i] = cin.nextInt();
13. Arrays.sort(a);
14. for (int i = 0; i < n; i++) System.out.print(a[i] + " ");
15. }
16. }

1、Arrays.sort()
                   （1）数字排序
                                int[] num =newint[]{4,2,3,5,-8,5,2,3};

Arrays.sort(num);

for(int i = 0; i < 8;i++)

System.out.print(num[i]+”,”);

输出结果：-8,2,2,3,3,4,5,5,

（2）字符串排序（先大写后小写原则）
           String[] s = new String[]{"Z","a","A","z"};

Arrays.sort(s);

for(int i = 0; i < 4;i++)

System.out.print(s[i] +",");

输出结果：A,Z,a,z,

（3）严格按照字母表排序（即忽略大小写）
          String[] s = new String[]{"Z","a","A","z"};

Arrays.sort(s,String.CASE_INSENSITIVE_ORDER);

for(int i = 0; i < 4;i++)

System.out.print(s[i] +",");

输出结果：a,A,Z,z,

（4）忽略大小写反向排序
          String[] s = new String[]{"Z","a","A","z"};

Arrays.sort(s,String.CASE_INSENSITIVE_ORDER);

Collections.reverse(Arrays.asList(s));

for(int i = 0; i < 4;i++)

System.out.print(s[i] +",");

输出结果：z,Z,A,a,

（5）对象的排序（类似于C中的结构体的排序）

package zoj3380;

import java.io.*;
import java.lang.reflect.Array;
import java.text.*;
import java.util.*;
import java.math.*;
public class Main
{
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        student[] students = new student[10];
        for (int i = 0; i < n; i++) {
            String s = scanner.next();
            int a = scanner.nextInt();
            int b = scanner.nextInt();
            students[i] = new student(a, b, s);
        }
//        int len = students.;
        Arrays.sort(students,0, n, new Acomparator());
//排序必须保证每个里面都有元素，要不然会报错。
//        Arrays.sort(students, new Acomparator());
        for(int i = 0; i < n; i++) {
            System.out.println(students[i].getA());
        }
    }
}

class  student {
    private int a, b;
    private String s;

    public student(int a, int b, String s) {
        this.a = a;
        this.b = b;
        this.s = s;
    }

    public int getA() {
        return a;
    }

    public void setA(int a) {
        this.a = a;
    }

    public int getB() {
        return b;
    }

    public void setB(int b) {
        this.b = b;
    }

    public String getS() {
        return s;
    }

    public void setS(String s) {
        this.s = s;
    }
}

class  Acomparator implements Comparator {

    @Override
    public int compare(Object o1, Object o2) {
        int a = ((student)o1).getA();
        int b = ((student)o2).getA();
        int c = a - b;
        if(c <= 0) return -1;
        else return 1;
    }
}