448. find all numbers disappeared in an array

Given an array of integers where 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements of [1, n] inclusive that do not appear in this array.

Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space.

Example:

Input:
[4,3,2,7,8,2,3,1]

Output:
[5,6]

意思就是从一组1 ≤ a[i] ≤ n的数组中，找到缺失的数

class Solution(object):
    def findDisappearedNumbers(self, nums):
    """
    :type nums: List[int]
    :rtype: List[int]
    """
    for num in nums:
        index = abs(num) - 1
        if nums[index] > 0:
            nums[index] *= -1
    return [i+1 for i, v in enumerate(nums) if v > 0]