balanced brackets c++

// program to implement balanced brackets using stack

#include <iostream>
#include<stack>

using namespace std;

int main() {

    int test;
    cin>>test;
    for(int i=0; i<test; i++){
        stack <char> s1;
        int flag = 0;
        string str;
        cin>>str;
        for (int j=0; j<str.length();j++){
            char s = str[j];
            if(s == '(' || s == '[' || s == '{'){
                s1.push(s);
            }
            else if(s == ')'){
                if (s1.size() > 0){
                    char temp = s1.top();
                    s1.pop();
                    if(temp != '('){
                        flag = true;
                        break;
                    }
                }
                else{
                    flag = true;
                    break;
                }
            }
            else if(s == ']'){
                if (s1.size() > 0){
                    char temp = (char) s1.top();
                    s1.pop();
                    if(temp != '['){
                        flag = true;
                        break;
                    }
                }
                else{
                    flag = true;
                    break;
                }
            }
            else if(s == '}'){
                if (s1.size() > 0){
                    char temp = (char) s1.top();
                    s1.pop();
                    if(temp != '{'){
                        flag = true;
                        break;
                    }
                }
                else{
                    flag = true;
                    break;
                }
            }
        }
        if(!flag && s1.size() == 0){
            cout<<"YES";
        }else{
            cout<<"NO";
        }
    }
}

/*

Sample Input

3
{[()]}
{[(])}
{{[[(())]]}}


Sample Output

YES
NO
YES
*/