（基础十）折半查找的实现（二分查找）

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/u013383042/article/details/53106339

首先必须明白，折半查找针对的是有序数组。

import java.util.*;

public class searchNum {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int key = sc.nextInt();

//      int[] arr = {5,2,1,9,8,6};//6个元素
//      int index = getIndex(arr,key);

        int[] arr = {2,6,7,9,15,18,35,39,40,56,66,89,100};//13个元素
//      int index = getIndex_2(arr,key);
//      int index = halfSearch(arr,key);
//      int index = halfSearch_2(arr,key);

        int index = Arrays.binarySearch(arr,key);//java提供了一个进行折半查找的函数。开发时使用这个。
        System.out.println("元素" + key + "在数组的第" + (index + 1) + "个位置！");
    }

    //折半查找---针对有序数组
    public static int halfSearch(int[] arr,int key){
        int min = 0;
        int max = arr.length - 1;
        int mid = (min + max) >> 1;

        while(arr[mid] != key){
            if(key > arr[mid])
                min = mid + 1;
            else if(key < arr[mid])
                max = mid - 1;

            if(min > max)
                return -1;

            mid = (min + max) >> 1;
        }

        return mid;
    }

    //折半查找的第二种方式
    public static int halfSearch_2(int[] arr,int key){
        int min = 0;
        int max = arr.length - 1;
        int mid;

        while(min <= max){
            mid = (min + max) >> 1;
            if(key > arr[mid])
                min = mid + 1;
            else if(key < arr[mid])
                max = mid - 1;
            else
                return mid;
        }

        return -1;
    }

    //针对有序数组才可使用
    public static int getIndex_2(int[] arr,int key){
        int min = 0;
        int max = arr.length - 1;
        int mid;

        while(min <= max){
            mid = (min + max) >> 1;

            if(key > arr[mid])
                min = mid + 1;
            else if(key < arr[mid])
                max = mid - 1;
            else
                return mid;
        }

        return -1;
    }

    //返回关键字key在数组arr中的位置，如果不存在则返回-1
    public static int getIndex(int[] arr, int key) {
        for(int i = 0; i < arr.length; i++){
            if(arr[i] == key){
                return i;
            }
        }
        return -1;
    }
}

原创：http://blog.csdn.net/u013383042/article/details/53106339