【AcWing】有效类型

#include<bits/stdc++.h>
using namespace std;
string str = "", s;
int flag = 1;
void input()
{
    if (cin >> s )
    {
        str += s;
        if ( s == "pair" )
        {
            str += '<';
            input();
            str += ',';
            input();
            str += '>';
        }
    }
    else flag = 0;
}
int main()
{
    int n;
    cin >> n;
    input();
    if (cin >> s )  flag = 0;
    if (!flag )   cout << "Error occurred\n";
    else    cout << str << endl;
}