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.
总结（思路）
计数 + 从小到大配对：
- 若
changed长度为奇数直接空数组； - 统计频次（或排序后双指针 / 哈希）。
- 按值从小到大遍历
x：需要用count[x]去匹配2x的count[2x]（特别注意x==0要偶数个）。 - 每匹配一次，将
x放入结果并减少对应频次；若不足则失败。
时间 O(n log n)（排序版），空间 O(n)。
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