LeetCode 448 Find All Numbers Disappeared in an Array

class Solution(object):
    def findDisappearedNumbers(self, nums):
        """
        :type nums: List[int]
        :rtype: List[int]
        """
        # https://leetcode.com/problems/find-all-numbers-disappeared-in-an-array/discuss/92956/Java-accepted-simple-solution
        res = []
        if nums:
            n = len(nums)
            for i in range(n):
                val = abs(nums[i]) - 1
                if nums[val] > 0:
                    nums[val] = -nums[val]
            for i in range(n):
                if nums[i] > 0:
                    res.append(i + 1)
        return res