algorithm notes: leetcode#442 find all duplicates in an array

class :
    def findDuplicates(self, nums):
        """
        :type nums: List[int]
        :rtype: List[int]
        """
        ans = []
        for num in nums:
            if nums[abs(num)-1] > 0:
                nums[abs(num)-1] *= -1
            else:
                ans.append(abs(num))
        return ans