Match Substring After Replacement LeetCode solution in C++

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class Solution {
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 public:
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  bool matchReplacement(string s, string sub, vector<vector<char>> &mappings) {
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    int m(size(s)), n(size(sub));
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    unordered_map<char, unordered_set<char>> mp;
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    auto doit = [&](int ind) {
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      for (int i = ind, j = 0; i < ind + n; i++, j++) {
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        if (s[i] == sub[j] or mp[sub[j]].count(s[i])) continue;
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        return false;
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      }
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      return true;
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    };
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    for (auto &mapping : mappings) mp[mapping[0]].insert(mapping[1]);
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    ;
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    for (int i = 0; i <= m - n; i++)
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      if (doit(i)) return true;
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    return false;
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  }
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};