136. Single Number

1 xor 1 = 0, 1 xor 0 = 1. So XOR operator will cancel a number if it appears twice. So this problem can be solve by xor all the numbers and the one left will be the one that appears once.

1:  class Solution {  
2:  public:  
3:    int singleNumber(vector<int>& nums) {  
4:      int single = 0;  
5:      for (int n : nums) {  
6:        single ^= n;  
7:      }  
8:      return single;  
9:    }  
10:  };