Decode String LeetCode solution in C++

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class Solution {
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 public:
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  string decodeString(string s) {
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    stack<char> st;
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    for (int i = 0; i < s.size(); i++) {
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      if (s[i] != ']') {
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        st.push(s[i]);
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      } else {
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        string curr_str = "";
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        while (st.top() != '[') {
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          curr_str = st.top() + curr_str;
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          st.pop();
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        }
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        st.pop(); // for '['
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        string number = "";
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        // for calculating number
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        while (!st.empty() && isdigit(st.top())) {
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          number = st.top() + number;
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          st.pop();
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        }
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        int k_time = stoi(number); // convert string to number
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        while (k_time--) {
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          for (int p = 0; p < curr_str.size(); p++) st.push(curr_str[p]);
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        }
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      }
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    }
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    s = "";
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    while (!st.empty()) {
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      s = st.top() + s;
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      st.pop();
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    }
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    return s;
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  }
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};