Approach: Bitwise ones/twos

def singleNumber(nums):
    ones = twos = 0
    for x in nums:
        ones = (ones ^ x) & ~twos
        twos = (twos ^ x) & ~ones
    return ones

Complexity: O(n) time, O(1) space.

Full Solution — all languages

class Solution:
    def singleNumber(self, nums):
        ones = twos = 0
        for x in nums:
            ones = (ones ^ x) & ~twos
            twos = (twos ^ x) & ~ones
        return ones

import java.util.*;
class Solution {
    public int singleNumber(int[] nums) {
        int ones = 0, twos = 0;
        for (int x : nums) {
            ones = (ones ^ x) & ~twos;
            twos = (twos ^ x) & ~ones;
        }
        return ones;
    }
}

#include <bits/stdc++.h> using namespace std;
class Solution {
    public: int singleNumber(vector<int>& nums) {
        int ones = 0, twos = 0;
        for (int x : nums) {
            ones = (ones ^ x) & ~twos;
            twos = (twos ^ x) & ~ones;
        }
        return ones;
    }
};

/**
 * @param {number[]} nums
 * @return {number}
 */ var singleNumber = function(nums) {
    let ones = 0, twos = 0;
    for (const x of nums) {
        ones = (ones ^ x) & ~twos;
        twos = (twos ^ x) & ~ones;
    }
    return ones;
};

Single Number II

Example 1

Example 2

Constraints