Single Number II

Given an array of integers, every element appears three times except for one. 

Find that single one.
Your algorithm should have a linear runtime complexity. Could you implement it 
without using extra memory?

Solution: Count the number of each bit.

 1 class Solution {
 2 public:
 3     // assume that integers are 32bits
 4     int singleNumber(int A[], int n) {
 5         int res = 0;
 6         for(int i = 0; i < 32; i++) {
 7             int count = 0;
 8             int bit = 1 << i;
 9             for(int j = 0; j < n; j++) {
10                 if(A[j] & bit) {
11                     count++;
12                 }
13             }
14             if(count % 3) {
15                 res = res | bit;
16             }
17         }
18         return res;
19     }
20 };