2007. Find Original Array From Doubled Array

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

 

    public int[] findOriginalArray(int[] A) {
        int n = A.length, i = 0;
        if (n % 2 == 1) return new int[0];
        int[] res = new int[n / 2];
        Map<Integer, Integer> count = new TreeMap<>();
        for (int a : A)
            count.put(a, count.getOrDefault(a, 0) + 1);
        for (int x : count.keySet()) {
            if (count.get(x) > count.getOrDefault(x + x, 0))
                return new int[0];
            for (int j = 0; j < count.get(x); ++j) {
                res[i++] = x;
                count.put(x + x, count.get(x + x) - 1);
            }
        }
        return res;
    }

https://leetcode.com/problems/find-original-array-from-doubled-array/discuss/1470959/JavaC%2B%2BPython-Match-from-the-Smallest-or-Biggest-100