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]

解题要点：

把input list转为set，此步骤可处理重复的数字。然后在0到list的长度里检测是否有消失的数字，如有，加进返回list。

class Solution(object):
    def findDisappearedNumbers(self, nums):
        """
        :type nums: List[int]
        :rtype: List[int]
        """
        nums_set = set(nums)
        res = []
        for i in range(0, len(nums)):
            if i+1 not in nums_set:
                res.append(i+1)
        return res