2007. Find Original Array From Doubled Array

Description

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

find-original-array-from-doubled-array.py

class Solution:
    def findOriginalArray(self, changed: List[int]) -> List[int]:
        N = len(changed)
        if N & 1: return []

        res = []
        counter = Counter(changed)

        for k in sorted(counter.keys(), reverse = 1):
            if k & 1: continue

            if k == 0 and counter[k] >= 2:
                res += [k] * (counter[k] // 2)
                continue

            half = k // 2
            if half > 0:
                v = min(counter[k], counter[half])
                counter[half] -= v

                res += [half] * v

        if len(res) != N // 2:
            return []

        return res