那个第二题 2[a1[b]]有没有做的呢？

我的是：为嘛通过率0  ^_^(勿喷，菜鸡)

#include<iostream>
#include<string>
#include<algorithm>
#include<sstream>
using namespace std;
int main()
{
    string str;
    cin >> str;
    int len = str.length();
    int startpos;
    int endpos;
    
    string res;
    int cnt;
    for (int i = len - 1; i >= 1; i--)
    {
        if (str[i] == '[' )
        {
            string tmp;
            startpos = i;
            for (int j = i+1; j <len-1; j++)
            {
                if (str[j]  >='a' && str[j] <= 'z' || str[j] >= 'A' && str[j] <= 'Z')
                    tmp += str[j];
                else
                    break;
            }
            string cntk;
            bool isexit = false;
            for (int k = i; k >= 0; k--)
            {
                if (str[k] >= '0' && str[k] <= '9')
                {
                    isexit = true;
                    cntk = str[k] + cntk;
                }
                else
                    if (isexit == true)
                        break;
                
            }
            stringstream ss;
            ss << cntk;
            ss >> cnt;

        

            tmp =  tmp +res;
            string s;
            res = s;
        

            for (int k = 0; k < cnt; k++)
            {
                res += tmp;
            }
        }
    }
    cout << res << endl;
    system("pause");
}