394

class Solution {
public:
    string decodeString(string s) {
        string ans;
        stack<string> str;
        stack<int> count;
        int num = 0;
        for(char c: s){
            if(isdigit(c)){
                num = num*10 + c -'0';
            }
            else if(isalpha(c)){
                ans += c;
            }
            else if(c == '['){
                count.push(num);
                str.push(ans);
                ans = "";
                num = 0;
            }
            else if(c == ']'){
                string tmp = ans;
                int c = count.top()-1;
                count.pop();
                for(int i=0; i<c; i++)
                    ans += tmp;
                ans = str.top()+ans;
                str.pop();
            }
        }
        return ans;
    }
};