Single Number

 1 class Solution {
 2 public:
 3     int singleNumber(int A[], int n) {
 4         
 5         unordered_map<int, int> singleMap;
 6         for(int i = 0; i < n; i++)
 7         {
 8             if(singleMap.find(A[i]) != singleMap.end())
 9                 singleMap[ A[i] ]++;
10             else
11                 singleMap[ A[i] ] = 1;
12         }
13         
14         for(unordered_map<int, int>::iterator it = singleMap.begin(); it != singleMap.end(); it++)
15         {
16             if(it->second != 2)
17                 return it->first;
18         }
19     }
20 };