java 常用算法

Posted 2021-12-24 zhizhiyin

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(nlogn)

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+" ");
		
	

　　

二分法查询-非迭代

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;