394. Decode String

class Solution {
public:
    string decodeString(string s) {
        string res = "";
        int length = s.size();
        stack<int> num;
        stack<string> result;
        int number = 0;
        for(int i = 0;i < length;i++){
            if(s[i] >= ‘0‘ && s[i] <= ‘9‘)
                number = number*10 + s[i] - ‘0‘;
            else if(s[i] == ‘[‘){
                num.push(number);
                number = 0;
                result.push(res);
                res.clear();
            }
            else if(s[i] == ‘]‘){
                int k = num.top();
                num.pop();
                for(int j = 0;j < k;j++)
                    result.top() += res;
                res = result.top();
                result.pop();
            }
            else
                res += s[i];
        }
        return res;
    }
};