Number of Valid Words for Each Puzzle LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> findNumOfValidWords(vector<string> &words, vector<string> &puzzles) {
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    unordered_map<int, int> maskCnt;
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    for (const auto &w : words) ++maskCnt[bitmask(w)];
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    vector<int> ans(puzzles.size(), 0);
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    for (int i = 0; i < puzzles.size(); i++) {
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      int mask, subMask = mask = bitmask(puzzles[i]), first = bitmask(puzzles[i].substr(0, 1));
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      do {
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        if ((first & subMask) == first && maskCnt.count(subMask)) ans[i] += maskCnt[subMask]; // ok
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      } while (subMask = (subMask - 1) & mask);
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    }
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    return ans;
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  }
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 private:
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  int bitmask(const string &word, int mask = 0) {
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    for (auto c : word) mask |= (1 << (c - 'a'));
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    return mask;
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  }
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};