Find element occuring once when all other are present thrice

Given an array of integers arr[] of length N, every element appears thrice except for one which occurs once.

Find that element which occurs once.

• Time : O(n)
• Space : O(1)

class Solution {
public:
    int singleElement(int arr[] ,int N) {
        vector<int> bi(32);
        for(int i = 0; i < N; i++) {
            for(int j = 0; j < 32; j++) {
                if(arr[i] & (1<<j)) bi[j]++;
            }
        }
        int res = 0;
        for(int i = 0; i < 32; i++)
            res |= (bi[i] % 3) << i;
        return res;
    }
};