二分查找

    import java.util.Scanner;  

    public class Main {  
        public static void main(String[] args) {  
            int num = 30;  
            int a[]={1, 8, 12, 15, 16, 21, 30, 35, 39};  
            String s = "";  
            if(find_num(a,num)==-1)System.out.println("没找到");  
            else System.out.println(num+"的位置在第"+find_num(a,num)+"个位置");  
        }  
        public static int find_num(int a[] , int num) {  
            int star = 0;  
            int end = a.length - 1;  
            while(star<=end) {              //千万不能star != end 不然少了 star == end 这种可能  
                int mid = (star+end)/2;  
                if(a[mid]==num)return mid;  //找到了  
                if(a[mid]<num) star = mid+1;  
                if(a[mid]> num)end  = mid-1;  
            }  
            return -1;  //找不到  
        }  
    }