Number of Pairs of Strings With Concatenation Equal to Target LeetCode solution in C++

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class Solution {
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 public:
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  int numOfPairs(vector<string> &nums, string target) {
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    unordered_map<string, int> freq;
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    for (auto num : nums)
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      if (num.size() < target.size()) freq[num]++;
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    int res = 0;
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    for (auto [s, frq] : freq) {
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      if (target.find(s) == 0) {
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        if (s + s == target)
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          res += frq * (frq - 1);
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        else
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          res += frq * freq[target.substr(s.size())];
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      }
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    }
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    return res;
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  }
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};