374. Guess Number Higher or Lower

A typical binary search.

1:  // Forward declaration of guess API.  
2:  // @param num, your guess  
3:  // @return -1 if my number is lower, 1 if my number is higher, otherwise return 0  
4:  int guess(int num);  
5:  class Solution {  
6:  public:  
7:    int guessNumber(int n) {  
8:      int l = 1, r = n, mid = 0, res = 0;  
9:      while (l <= r) {  
10:        mid = l + (r-l) /2;  
11:        res = guess(mid);  
12:        if (res == 0) return mid;  
13:        else if (res == 1) l = mid + 1;  
14:        else r = mid - 1;  
15:      }  
16:      return mid;  
17:    }  
18:  };