要明白二分查找是個(gè)什么思想.個(gè)人比較熟悉,不寫二分思想說明.
一定要注意:使用二分之前,一定要排好序,要不然怎么進(jìn)行二分查找呢. 具體看代碼:(這里是用for循環(huán)實(shí)現(xiàn)的,后面的代碼是用遞歸實(shí)現(xiàn).)

package com.qf.demo2;

public class Test3 {

    public static void main(String[] args) {
        int[] a= {1,4,7,9,10,13,18,19};
        int index = binarySearch(a, 111);
        System.out.println(index);
    }
    public static int binarySearch(int[] a,int b){
        
        int minIndex = 0;
        int maxIndex = a.length-1;
        
        for(int i = 0; i<a.length/2+1;i++){
            int middleIndex = (minIndex+maxIndex)/2;    
            if(a[middleIndex] == b){
                return middleIndex;
            }else if(a[middleIndex]>b){// 左邊的情況  , 需要改變的是最大的下標(biāo)
                maxIndex = middleIndex-1;
            }else if(a[middleIndex]<b){// 右邊的情況, 需要改變的是最小的下標(biāo)
                minIndex = middleIndex+1;
            }
        }
        return -1;
    }
}

package com.qf.demo2;

import java.util.Arrays;

public class Ef {
    public static void main(String[] args) {
        int[] a = {9,8,1,4,2,6};
        //先排序
        Arrays.sort(a);
        int index = ef(a,2,0,5);
        System.out.println(index);
    }
    static int ef(int[] a,int key,int max,int min){
        int mid = (min+max)/2;
        if(a[mid] == key){
            return mid;
        }else if(a[mid] > key){
            return ef(a, key, min, mid-1);
        }else{
            return ef(a, key, mid+1, max);
        }
    }

}