LeetCode_Single Number II

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

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

class Solution {
public:
int singleNumber(int A[], int n) {
// Note: The Solution object is instantiated only once and is reused by each test case.
int res = 0;
int x, sum;

for(int i = 0; i < 32; ++i){

sum = 0;
x = (1<<i);

for(int j = 0; j< n; ++j)
if(A[j]&x)
++sum;

sum = sum%3;

if(sum == 1)
res |= x;
}

return res;
}
};