二分法

 1 #include<stdio.h>
 2 
 3 int binsearch(int x,int a[],int n)
 4 {
 5     int low,high,mid;
 6 
 7     low=0,high=n-1;
 8 
 9     while(low<=high)
10     {
11         mid=(low+high)/2;
12         if(x<a[mid]){
13             high=mid-1;
14         }
15         else if(x>a[mid]){
16             low=mid+1;
17         }
18         else{
19             return mid;
20         }
21     }
22     return -1;
23 }