decode-string.cpp

class Solution {
public:
    string decodeString(string s) {
        stack<char> st;
        for (char c : s) {
            if (c != ']') {
                st.push(c);
            } else {
                string curr;
                while (st.top() != '[') {
                    curr = st.top() + curr;
                    st.pop();
                }
                st.pop();
                string number = "";
                while (!st.empty() && isdigit(st.top())) {
                    number = st.top() + number;
                    st.pop();
                }
                int n = stoi(number);
                while (n--) {
                    for (char d : curr) {
                        st.push(d);
                    }
                }
            }
        }
        s = "";
        while (!st.empty()) {
            s = st.top() + s;
            st.pop();
        }
        return s;
    }
};