An integer array original is transformed into a doubled array changed by appending twice the value of every element in original, and then randomly shuffling the resulting array.

Given an array changed, return original if changed is a doubled array. If changed is not a doubled array, return an empty array. The elements in original may be returned in any order.

Example 1:

Input: changed = [1,3,4,2,6,8]
Output: [1,3,4]
Explanation: One possible original array could be [1,3,4]:
- Twice the value of 1 is 1 * 2 = 2.
- Twice the value of 3 is 3 * 2 = 6.
- Twice the value of 4 is 4 * 2 = 8.
Other original arrays could be [4,3,1] or [3,1,4].


Example 2:

Input: changed = [6,3,0,1]
Output: []
Explanation: changed is not a doubled array.


Example 3:

Input: changed = [1]
Output: []
Explanation: changed is not a doubled array.


Constraints:

• 1 <= changed.length <= 105
• 0 <= changed[i] <= 105

## Solution 1: Multiset

Start from the smallest number x, erase one x * 2 from the set.

Time complexity: O(nlogn)
Space complexity: O(n)

## Solution 2: Hashtable

Time complexity: O(max(nums) + n)
Space complexity: O(max(nums))

## C++/Hashtable

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