[LeetCode] 448. Find All Numbers Disappeared in an Array

https://leetcode.com/problems/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]

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關鍵在不能用額外的空間 和 run time O(n)

這裡用一個 "字典", 兩個 loop 做快速搜索已出現過的 數字