java 常用算法和一些题目

选择排序，复杂度O（n²）

package com.example.demo;

import org.junit.Test;
/**
 * 选择排序
 * @author zhzh.yin
 *
 */
public class HTest {
	@Test
	public void testMethod(){
		Integer [] numList = {1,2,2,2,1,4,5,2,5,3,9,6};
		for(int i=0;i<numList.length-1;i++){
			int max = numList[i];
			for(int j = i+1;j<numList.length;j++){
				if(numList[i]<numList[j]){
					max = numList[j];
					numList[j]=numList[i];
					numList[i]=max;
				}
			}
		}
		for(int num:numList){
			System.out.print(numList[num]+" ");
		}
	}
}

　　

冒泡排序 复杂度 O(n2)

package com.example.demo;

import org.junit.Test;
/**
 * 冒泡排序
 * @author zhzh.yin
 *
 */
public class FTest {
	@Test
	public void testMethod(){
		Integer[]numList = {1,2,1,1,1,8,3,5,1};
		if(numList.length==1){
			System.out.println("-1");
		}else if(numList.length==2){
			String string ="";
			System.out.println(
					numList[1]>numList[2]?
					numList[1]+""+numList[0]:numList.toString());;
		}
		int temp = 0;
		for(int i =0;i<numList.length-1;i++){
			for(int j=i+1;j<numList.length;j++){
				if(numList[i]<numList[j]){
					temp=numList[i];
					numList[i]=numList[j];
					numList[j]=temp;
				}
			}
		}
		for(int num :numList){
			System.out.print(num+" ");
		}
	}
}

　　

快速排序O（nlogn）

@Test
    public void speedSortTest(){
        Integer []numList={2,3,7,9,5,10,1,4};
        speedSort(numList,0,numList.length-1);
       for(int time:numList){
           System.out.print(time+" ");
       }
    }
    public void speedSort(Integer []numList,int lower,int higher){

        if(lower>=higher){
            return;
        }
        int temp=0;
        int icon = numList[lower];

        int indexofLeft = lower;
        int indexofRight = higher;
        while(indexofLeft<indexofRight){
            while(icon<=numList[indexofRight]&&indexofLeft<indexofRight){
                    indexofRight--;
            }
            while(icon>=numList[indexofLeft]&&indexofLeft<indexofRight){
                indexofLeft++;
            }

            if(indexofLeft<indexofRight){
                temp = numList[indexofRight];
                numList[indexofRight]=numList[indexofLeft];
                numList[indexofLeft]=temp;
            }
            for(int num:numList){
                System.out.print(num+" ");
            }
            System.out.println();
        }
        temp=numList[indexofLeft];
        numList[indexofLeft]=numList[lower];
        numList[lower]=temp;

        speedSort(numList,lower,indexofLeft-1);
        speedSort(numList,indexofRight+1,higher);
    }

　　

二分法查询-非迭代O（logn）

http://hedengcheng.com/?p=595

import org.junit.Test;
/**
 * 找到数字N，适用于有多个重复数字，使用部分math原生方法向上取整/向下取整
 * middle=min+(max-min)/2 算法更加准确
 * 返回-1为没有找到
 * @author zhzh.yin
 *
 */
public class CTest {
	@Test
	public void testMethod(){
		Integer[] num ={1,2,3,3,3,4,5,5,5,5,5,6,6,6,7,10};
		System.out.println("the num is at "+selectPosition(num,8));
	}
	public int selectPosition(Integer [] numList,int num){
		int min=0;
		int max=numList.length-1;
		int middle = -1;
		while(min<=max){
			middle = min+(max-min)/2;
			System.out.println("min--"+min+",max--"+max+",middle--"+middle);
			if((min==middle||max==middle)&&numList[middle]!=num){
				middle = -1;
				break;
			}
			if(numList[middle]<num){
				min=(int) Math.floor(middle);
			}else if(numList[middle]>num){
				max=(int) Math.ceil(middle);
			}else{
				break;
			}
		}
		return middle;
	}
}

　　

迭代+复杂化二分法

package com.example.demo;

import org.junit.Test;

/**
 * 找出数组中最后一个6(或者8)的位置,复杂度O(nlogn)，用迭代实现
 *
 * @author zhzh.yin flag==true 获取最右的坐标
 *
 */
public class CTest {
	@Test
	public void testMethod() {
		Integer[] num = { 1, 2, 3, 3, 3, 4, 5, 5, 5, 5, 5, 6, 6, 6, 7, 10 };
		System.out.println("hello,position is " + selectPosition(num, 10, 0, num.length - 1, false));
	}

	public int selectPosition(Integer[] numList, int num, int min, int max, Boolean flag) {
		int middle = min + (max - min) / 2;
		System.out.println("min-" + min + ",max-" + max + ",middle-" + middle);
		if (min < max - 1) {
			if (numList[middle] < num) {
				return selectPosition(numList, num, (int) Math.floor(middle), max, flag);
			} else if (numList[middle] > num) {
				return selectPosition(numList, num, min, (int) Math.ceil(middle), flag);
			} else if (numList[middle] == num && (min != max && min != max - 1)) {
				if (flag) {
					return selectPosition(numList, num, (int) Math.ceil(middle), max, flag);
				} else {
					return selectPosition(numList, num, min, (int) Math.floor(middle), flag);
				}
			}
		} else{
			if (numList[min] != num && numList[max] != num) {
				middle = -1;
				return middle;
			} else if (numList[min] == numList[max]) {
				return flag.equals(true) ? max : min;
			} else if (numList[min] == num) {
				return min;
			} else {
				return max;
			}
		}
		return middle;
	}
}

顺时针打印矩阵

递归

import org.junit.Test;

/**
 * 顺时针打印矩阵
 * @author user
 *
 */
public class ATest {
//顺时针打印一个矩阵

    @Test
    public void test(){
        int[][] num = new int[100][100];
        int n = 6;
        int count =1;

        for(int i=0;i<n;i++){
            for(int j =0;j<n;j++){
                num[i][j]=count++;
            }
        }
        output(num,0,n-1);
    }
    /**
     *
     * @param num
     * @param start
     * @param end
     */
    public void output(int[][] num,int start,int end){
        if(start>end || end<=0)return;
        for(int i=start;i<=end;i++){
            System.out.println(num[start][i]);
        }
        for(int i=start+1;i<=end;i++){
            System.out.println(num[i][end]);
        }
        for(int i=end-1;i>=start;i--){
            System.out.println(num[end][i]);
        }
        for(int i=end-1;i>start;i--){
            System.out.println(num[i][start]);
        }
        output(num,start+1,end-1);
    }
}

　　

找出出现次数最多的

先recordNum 再从hashMap里面找出value最大的

package com.example.demo;

import java.util.HashMap;
import java.util.Map;

import org.junit.Test;
/**
 * 找出出现次数最多的数值
 * @author zhzh.yin
 *
 */
public class BTest {
	@Test
	public void TestMethod(){
		Integer []numList = {1,1,1,-1,2,3,44,4,5,6,9,9,9,9,9,8,7,6,5};
//		chooseMax(recordNum(numList));
		System.out.println(chooseMax(recordNum(numList),numList[0]));
	}

	public HashMap <Integer,Integer> recordNum (Integer [] numList){
		HashMap<Integer, Integer> map=new HashMap<>();
		for(int num:numList){
			if(map==null){
				map.put(num, 1);
				continue;
			}
			if(map.containsKey(num)){
				map.put(num, map.get(num)+1);
			}else{
				map.put(num, 1);
			}
		}
		return map;
	}
	public String chooseMax(HashMap<Integer, Integer>map,int num){
		String string = "";
		int key = num;
		int time = map.get(key);
		for(int keyActually :map.keySet()){
			if(map.get(keyActually)>time){
				time=map.get(keyActually);
				key = keyActually;
			}
		}
		string= "出现次数最多的数是"+key+" "+time+" "+map.toString();
		return string;
	}
}

　　

把AB文件字符合并重排，输出到C

package com.example.demo;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.junit.Test;
/**
 * 把AB文件中的字符重排，输入到C
 * @author zhzh.yin
 *
 */
public class ETest {
	@Test
	public void TestMethod() throws IOException{
		FileReader filea = new FileReader("C:\\Users\\user\\Downloads\\a.txt");
		BufferedReader ina = new BufferedReader(filea) ;
		FileReader fileb = new FileReader("C:\\Users\\user\\Downloads\\b.txt");
		BufferedReader inb = new BufferedReader(fileb) ;
		String line ;
		char[]arr;
		List<Character> list = new ArrayList<>();
		BufferedWriter writerc= new BufferedWriter(new FileWriter("C:\\Users\\user\\Downloads\\c.txt"));
		while((line=ina.readLine())!=null){
			arr =line.toCharArray();
			for(int i=0;i<arr.length;i++){
				list.add(arr[i]);
			}
		}
		ina.close();
		while((line=inb.readLine())!=null){
			arr =line.toCharArray();
			for(int i=0;i<arr.length;i++){
				list.add(arr[i]);
			}
		}
		inb.close();

		Object[]array = list.toArray();
		Arrays.sort(array);
		for(Object object:array){
			writerc.write((char)object);
		}
		writerc.close();
		FileReader filec = new FileReader("C:\\Users\\user\\Downloads\\c.txt");
		BufferedReader read = new BufferedReader(filec);
		while((line = read.readLine())!=null){
			System.out.println(line);
		}
		read.close();
	}

}

　　

找出出现2次的值

package com.example.demo;

import java.util.HashMap;
import org.junit.Test;
/**
 * 找出出现2次的数值
 * @author zhzh.yin
 *
 */
public class GTest {
	@Test
	public void testMethod(){
		Integer [] numList = {1,2,2,2,3,4,5,5,6,7,7};
		HashMap<Integer,Integer> numMap = new HashMap();
		for(int num :numList){
			if (!numMap.containsKey(num)){
				numMap.put(num, 1);
			}else {
				numMap.put(num, numMap.get(num)+1);
			}
		}
		System.out.println(numMap);
		for(HashMap.Entry<Integer, Integer>entry :numMap.entrySet()){
			if(entry.getValue()==2){
			System.out.print(entry.getKey()+" ");}
		}
	}
}