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) {

int retVal = 0;

for (int idx=0; idx<32; idx++)
{
int bit_count = 0;
for (int i=0; i<n; i++)
{
if (A[i] & (0x1<<idx))
{
bit_count++;
}
}

if (bit_count%3 == 1)
{
retVal |= (0x1<<idx);
}

}

return retVal;
}
};