You can use a hash map to record the frequencys. Or you can use bit operation.

x ^ x = 0. So the only left is the one.

 

1 class Solution { 2 public: 3     int singleNumber(int A[], int n) { 4         int result = 0; 5         for (int i = 0; i < n; i++) { 6             x ^= A[i]; 7         } 8         return result; 9     }10 };