394. 字符串解码/C++

string decodeString(string s) {
    int num=0;
    string str="";
    stack<int> numstk;
    stack<string> strstk;
    for(int i=0;i<(int)s.size();++i){
        if(s[i]>='0'&&s[i]<='9'){
            num=10*num+s[i]-'0';
        }
        else if(s[i]>='a'&&s[i]<='z'||s[i]>='A'&&s[i]<='Z'){
            str+=s[i];
        }
        else if(s[i]=='['){
            numstk.push(num);
            num=0;
            strstk.push(str);
            str="";
        }
        else if(s[i]==']'){                
            int time=numstk.top();
            numstk.pop();
            
            for(int i=0;i<time;++i)
                strstk.top()+=str;
            
            str=strstk.top();
            strstk.pop();
        }
    }
    return str;
}