394. 字符串解码

class Solution 
{
public:
    string decodeString(string s) 
    {
        int len = s.size();
        int num = 0;
        stack<int> numstack;
        stack<string> strstack;
        string cur = "";
        string result = "";
        for(int i=0; i<len; ++i)
        {
            if(isdigit(s[i])) num = 10*num + s[i] - '0';
            else if(s[i] == '[')
            {
                numstack.push(num);
                strstack.push(cur);//同时放入，保持两个栈的大小相同
                num = 0;
                cur.clear();
            }
            else if(isalpha(s[i])) cur += s[i];
            else if(s[i] == ']')
            {
                int k = numstack.top();
                numstack.pop();
                for(int j=0; j<k; ++j) strstack.top() += cur;
                cur = strstack.top();
                strstack.pop();
            }
        }
        result = result + cur;
        return result;     
    }
};