LeetCode 448 Find All Numbers Disappeared in an Array

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本文介绍了一种高效查找数组中缺失数字的方法，不使用额外空间且时间复杂度为O(n)。通过对数组进行遍历，利用数组元素作为标记，最终找出未出现过的数字。

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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]

对数组遍历，如果nums[nums[i]]大于0 就把他乘以-1，本质是做标记哪些数字出现过，最后把值为正的索引输出

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