Cracking the Safe LeetCode solution in C++

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class Solution {
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 public:
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  int total;
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  unordered_set<string> st;
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  bool func(int i, int k, int n, string &s) {
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    if (i == total) return 1;
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    for (int j = 0; j < k; j++) {
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      s.push_back(j + '0');
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      if (s.size() >= n) {
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        string a = s.substr(s.size() - n);
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        if (st.find(a) == st.end()) {
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          st.insert(a);
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          if (func(i + 1, k, n, s)) return 1;
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          st.erase(a);
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        }
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      } else if (func(i, k, n, s))
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        return 1;
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      s.pop_back();
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    }
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    return 0;
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  }
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  string crackSafe(int n, int k) {
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    total = pow(k, n);
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    string s;
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    func(0, k, n, s);
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    return s;
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  }
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};