java程序中“两分法”查找数据

package Daniel;

import java.util.Scanner;

public class DoSearch {

	public static int bisSearch(int target){
		int[] a={2,3,5,7,12,15,18,36,45,67,74,87,88};
		int btm=0;
		int top=a.length;
		int mid=(btm+top)/2;
		while(a[mid]!=target){
			int old_mid = mid;
			if(a[mid]>target){
				top=mid;
			}else{
				btm=mid;
			}
			mid=(btm+top)/2;
			int new_mid=mid;
			
			if(old_mid==new_mid){
				return -1;
			}
		}
		
		return mid;
	}
	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		System.out.println("请输入你想查找的数： ");
		int a=sc.nextInt();
		int index= bisSearch(a);
		System.out.println(index);
		if (index==-1){
			System.out.println("序列中没有您要查找的数，请确认输入正确。");
		}else{
			System.out.println("所查找的数所在位置为：第"+(index+1)+"位");
		}
	}

}

 

这里关键是while(){}循环语句的使用：

while(a[mid]!=target){
			int old_mid = mid;
			if(a[mid]>target){
				top=mid;
			}else{
				btm=mid;
			}
			mid=(btm+top)/2;
			int new_mid=mid;
			
			if(old_mid==new_mid){
				return -1;
			}

 

这里 mid的值 不断的改变 所以a[mid]不断的变化...来促使while（）{}的循环和终止... index是一串有序的数值..