401. Binary Watch

The idea behind is to enumerate all possible combinations and count the bit of 1. If the bits number matches the input, then output the time.

1:  class Solution {  
2:  public:  
3:    vector<string> readBinaryWatch(int num) {  
4:      vector<string> res;  
5:      for (int h = 0; h < 12; h++) {  
6:        for (int m = 0; m < 60; m++) {  
7:          if (countBits((h << 6) | m) == num) {  
8:            res.push_back(to_string(h) + (m < 10 ? ":0" : ":") + to_string(m));  
9:          }  
10:        }  
11:      }  
12:      return res;  
13:    }  
14:    int countBits(int n) {  
15:      int count = 0;  
16:      while (n) {  
17:        n &= n-1;  
18:        count++;  
19:      }  
20:      return count;  
21:    }  
22:  };