Find Original Array From Doubled Array LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> findOriginalArray(vector<int> &changed) {
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    unordered_map<int, int> freq;
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    for (auto num : changed) freq[num]++;
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    sort(changed.begin(), changed.end());
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    vector<int> res;
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    for (auto num : changed) {
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      if (freq[num] && freq[num * 2]) {
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        freq[num]--;
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        freq[num * 2]--;
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        res.push_back(num);
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      }
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    }
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    for (auto [a, b] : freq)
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      if (b) return {};
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    return res;
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  }
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};