main.py

#!/usr/bin/env python3

# O(n) time, O(1) space

# [1, 2, 1, 2, 3, 4, 4] -> 3
def single_number_twice(nums):
    s = 0
    for num in nums:
        s ^= num
    return s

# [1, 2, 1, 2, 3, 4, 4, 1, 2, 4] -> 3
def single_number_thrice(nums):
    # 1st, 4th, 7th...
    ones = 0
    # 2nd, 5th, 8th...
    twos = 0
    # "Disable" the 3rd time bits
    common = 0
    for num in nums:
        twos |= num & ones
        ones ^= num
        # ones & twos - bits that we saw 3 times, so we get the OPPOSITE mask.
        common = ~(ones & twos)
        # Turn off those bits.
        ones &= common
        twos &= common
    return ones

# [1, 2, 1, 2, 3, 4, 4, 5] -> [3, 5] / [5, 3]
def duo_twice(nums):
    s = 0
    for num in nums:
        s ^= num # 3 ^ 5
    s &= -s
    s_on = 0
    s_off = 0
    for num in nums:
        if num & s:
            s_on ^= num
        else:
            s_off ^= num
    return [s_on, s_off]

print(duo_twice([1, 2, 1, 2, 3, 4, 4, 5]))