Maximum Score From Removing Substrings LeetCode solution in C++

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class Solution {
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 public:
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  int helper(string &str, char a, char b) {
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    int count = 0;
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    stack<char> st;
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    for (int i = 0; i < str.length(); i++) {
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      if (!st.empty() && str[i] == b && st.top() == a) {
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        st.pop();
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        count++;
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      } else {
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        st.push(str[i]);
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      }
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    }
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    str = "";
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    while (!st.empty()) {
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      str += st.top();
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      st.pop();
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    }
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    reverse(str.begin(), str.end());
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    return count;
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  }
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  int maximumGain(string s, int x, int y) {
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    int ca = 0, cb = 0;
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    if (x > y) {
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      ca = helper(s, 'a', 'b');
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      cb = helper(s, 'b', 'a');
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    } else {
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      cb = helper(s, 'b', 'a');
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      ca = helper(s, 'a', 'b');
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    }
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    return ca * x + cb * y;
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  }
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};