Lexicographically Smallest String After Applying Operations LeetCode solution in C++

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class Solution {
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 public:
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  void dfs(string &s, string &small, int a, int b, unordered_map<string, string> &memo) {
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    if (memo.count(s)) return;
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    string res = s, str = res;
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    memo[s] = res;
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    rotate(str.begin(), str.begin() + b, str.end()); // rotate current string
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    dfs(str, small, a, b, memo);
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    if (memo[str] < res) res = memo[str];
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    for (int i = 0; i < 9; i++) { // or add it
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      for (int j = 1; j < s.length(); j += 2) s[j] = '0' + ((s[j] - '0') + a) % 10;
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      if (s < res) res = s;
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      dfs(s, small, a, b, memo);
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    }
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    if (small > res) small = res;
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    memo[s] = res;
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  }
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  string findLexSmallestString(string s, int a, int b) {
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    unordered_map<string, string> memo;
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    string res = s;
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    dfs(s, res, a, b, memo);
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    return res;
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  }
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};