137. Single Number II

Description

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

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

Solution

Bit-manipulation, time O(n), space O(1)

class Solution {
    public int singleNumber(int[] nums) {
        int single = 0;
        
        for (int i = 0; i < 32; ++i) {
            int sum = 0;
            
            for (int n : nums) {
                sum += (n >> i) & 1;
            }
            
            sum %= 3;
            single |= sum << i;
        }
        
        return single;
    }
}