K-Similar Strings LeetCode solution in C++

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class Solution {
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 public:
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  unordered_map<string, int> m;
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  int solve(string &s1, string &s2, int i) {
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    if (i == s1.length()) return 0;
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    if (m.find(s1) != m.end()) return m[s1];
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    if (s1[i] == s2[i]) return m[s1] = solve(s1, s2, i + 1);
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    int ans = 1e5;
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    for (int j = i + 1; j < s1.length(); j++) {
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      if (s1[j] == s2[i]) {
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        swap(s1[j], s1[i]);
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        ans = min(ans, 1 + solve(s1, s2, i + 1));
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        swap(s1[j], s1[i]);
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      }
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    }
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    return m[s1] = ans;
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  }
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  int kSimilarity(string s1, string s2) {
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    return solve(s1, s2, 0);
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  }
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};