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