137. Single Number II

https://leetcode.com/problems/single-number-ii/

Python

Hash Set

class Solution:
    def singleNumber(self, nums: List[int]) -> int:
        m1, m2 = set(), set()

        for num in nums:
            if num not in m1:
                m1.add(num)
                continue

            if num not in m2:
                m2.add(num)
                continue

            m1.remove(num)
            m2.remove(num)

        return m1.pop()

Bit Manipulation

• ~ bitwise NOT
• ^ xor

class Solution:
    def singleNumber(self, nums: List[int]) -> int:
        ones, twos = 0, 0

        for num in nums:
            ones = (ones ^ num) & ~twos
            twos = (twos ^ num) & ~ones

        return ones