Check If Word Is Valid After Substitutions LeetCode solution in C++

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class Solution {
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 public:
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  bool isValid(string s) {
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    stack<char> st;
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    for (int i = 0; i < s.length(); i++) {
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      if (s[i] == 'a')
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        st.push(s[i]);
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      else if (s[i] == 'b') {
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        if (st.empty() || st.top() != 'a')
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          return false;
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        else
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          st.push(s[i]);
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      } else if (s[i] == 'c') {
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        if (st.empty() || st.top() != 'b')
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          return false;
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        else {
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          for (int i = 0; i < 2; i++) st.pop();
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        }
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      }
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    }
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    return st.size() == 0;
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  }
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};