137. Single Number II

The comment in the code explains the solution clearly.

1:  class Solution {  
2:  public:  
3:    int singleNumber(vector<int>& nums) {  
4:      int one = 0, two = 0, three = 0;  
5:      for (int i = 0; i < nums.size(); i++) {  
6:        two |= one & nums[i]; // get the bits that appears twice  
7:        one ^= nums[i]; // cancel the bits that appears twice  
8:        three = one & two; // get the bits that appears three times  
9:        one ^= three; // cancel the bits that appears three times  
10:        two ^= three; // cancel the bits that appears three times  
11:      }  
12:      return one;  
13:    }  
14:  };