括号配对

这是一段   WA代码...........
#include <iostream>
#include <stack>
#include<string>
using namespace std;
void main()
{
    stack<char> c;
    int T,len;
    cin >> T;
    getchar();
    string str;
    while (T--)
    {
        getline(cin, str);
        len=str.length();
        if (len % 2 != 0)
        {
            cout << "No" << endl;
            continue;
        }
        else if(len == 0)
        {
            cout << "Yes" << endl;
            continue;
        }
        if (str[0] == ')' || str[0] == ']')
        {
            cout << "No" << endl;
            continue;
        }
        for (int i = 0; i < len; i++)
        {
            if (str[i] == '(' || str[i] == '[')
                c.push(str[i]);
            else
            {
                if (str[i] == ')' && c.top()=='(' || str[i] == ']' && c.top() == '[')
                    c.pop();
                else
                {
                    cout << "No" << endl;
                    continue;
                }
            }
            if (i == len - 1)
            {
                if (c.size())
                    cout << "No" << endl;
                else
                    cout << "Yes" << endl;
            }
        }
    }
}

AC代码：
#include<iostream>
#include<stdio.h>
#include<string>
using namespace std;
string del(string str,int n)
{
    if (str == "")
        return str;
    for (int i = n;i < str.size() - 1;i++)
    {
        if ((str[i] == '('&&str[i + 1] == ')')
            || (str[i] == '['&&str[i + 1] == ']'))
        {
            str.erase(i, 2);
            i = i > 0 ? i - 1 : i;
            return del(str, i);
        }
    }
    return str;
}
int main()
{
    int T;
    cin >> T;
    getchar();
    while (T--)
    {
        string str;
        getline(cin, str);
        if (del(str,0) == "")
            cout << "Yes" << endl;
        else
            cout << "No" << endl;
    }
}