LeetCode: 442. Find All Duplicates in an Array

O(N), O(1) Solution by using input array as a memory.

class Solution:
    def findDuplicates(self, nums: List[int]) -> List[int]:
        ret = []
        for i, num in enumerate(nums):
            if nums[abs(num) - 1] < 0:
                ret.append(abs(num))
            nums[abs(num) - 1] *= -1
        return ret