二分查找

int s(int l,int r,int k)//二分查找
 76 {
 77     int mid;
 78      while(l<=r)
 79      {
 80          mid=(l+r)>>1;
 81          if(k<x[mid]) r=mid-1;
 82          else if(k>x[mid]) l=mid+1;
 83          else return mid;
 84      }
 85      return -1;
 86 }