数组元素的查找（折半查找）

/*
     数组的查找 
     折半查找：前提是该数组是有序的数组
 */

import java.util.Scanner;

public class arrLookup{
    public static void main(String[] args){
        int[] arr = {1,4,6,8,34,56,78};
        System.out.println("Please enter the number you want to find:");
        Scanner in = new Scanner(System.in);
        int number = in.nextInt();
        
        //halfSearch
        int index = halfSearch(arr, number);
        System.out.println("index = "+index);
        //halfsearch_2
        int index_2 = halfSearch_2(arr, number);
        System.out.println("index_2 = "+index_2);
        
        //练习：将元素有序插入数组
        Scanner in_1 = new Scanner(System.in);
        int number_1 = in_1.nextInt();
        int index_3 = getIndex(arr, number_1);
        System.out.println("index_3 = "+index_3);
        
    
    }
    
    //折半查找
    public static int halfSearch(int[] arr, int key){
        int min = 0;
        int max = arr.length-1;
        int mid = (min+max)/2;
        //判断是否找到key
        while(key != arr[mid]){
            if(key > arr[mid])    //key大于中间值，把中间值后一个角标赋值给min，max不变
                min = mid+1;
            else if(key < arr[mid])    //key小于中间值，把中间值前一个角标赋值给max，min不变
                max = mid-1;
            
            if(min > max)    //如果min>max，说明该数组内没有key
                return -1;    //该数组没有key
            
            mid = (min+max)/2;    //折半
        }
        return mid;    
    }
    
    //第二种折半查找
    public static int halfSearch_2(int[] arr, int key){
        int min = 0;
        int max = arr.length-1;
        int mid;
        //判断min是否小于等于max
        while(min <= max){
            mid = (min+max)>>1;
            if(key > arr[mid])
                min = mid+1;
            else if(key < arr[mid])
                max = mid-1;
            else if(key == arr[mid])    //找到key，返回角标
                return mid;
        }
        return -1;
        
    }
    
    //练习:将元素有序插入数组
    public static int getIndex(int[] arr, int key){
        int min = 0;
        int mid;
        int max = arr.length-1;
        while(min <= max){
            mid = (min+max)>>1;
            if(key > arr[mid])
                min = mid+1;
            else if(key < arr[mid])
                max = mid-1;
            else if(key == arr[mid])
                return min;
        }
        return min;
    }
}