## Description

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]


## Solutions

### 1. Index Trick

将每个数位上的数当做索引（去其正数值减 1），通过该数索引找到对应的数，然后将对应索引位置上的数变成负数。

# Time: O(n)
# Space: O(1)
class Solution:
def findDisappearedNumbers(self, nums: List[int]) -> List[int]:
n = len(nums)
for i in range(n):
idx = abs(nums[i]) - 1
nums[idx] = - abs(nums[idx])
return [i + 1 for i in range(n) if nums[i] > 0]

# 34/34 cases passed (412 ms)
# Your runtime beats 40.21 % of python3 submissions
# Your memory usage beats 46.43 % of python3 submissions (20.5 MB)


### 2. Set

这个就不符合不要额外空间的要求，但是从速度上来讲，这个是最快的。在数据量不大的情况下，我们可以尝试使用这种办法。

# Time: O(n)
# Space: O(1)
class Solution:
def findDisappearedNumbers(self, nums: List[int]) -> List[int]:
n = len(nums)
nums_set = set(nums)
res = []
for i in range(1, n+1):
if i not in nums_set:
res.append(i)
return res

# 34/34 cases passed (356 ms)
# Your runtime beats 94.57 % of python3 submissions
# Your memory usage beats 7.14 % of python3 submissions (22.8 MB)
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