binary search

 1 int binarySearch(int a[], int n, int x){
 2     int lo = 0;
 3     int hi = n-1;
 4     while(lo<=hi){
 5         int mid = lo + (hi-lo)/2;
 6         if (a[mid] == x) return mid;
 7         else if(a[mid]<x)
 8             lo = mid + 1;
 9         else hi = mid - 1;
10     }
11     return -1;  //not found;
12 }