Balanced Brackets {[(una string)]} HD

#include <iostream>
#include <vector>

using namespace std;

string s;

bool cancelan (char a, char b)
{
    if (a=='(' && b==')')
        return true;
    if (a=='[' && b==']')
        return true;
    if (a=='{' && b=='}')
        return true;
    return false;
}

bool balanced ()
{
    string guard;
    guard+=s[0];

    for(int i=1; i<s.size(); i++)
    {
        char ult=guard.back();
        char car=s[i];
        if( cancelan(ult,car) )
            guard.pop_back();
        else
            guard+=car;
    }

    if(guard.size()==0) ///si no me qued0 alg0 por conectar
        return true;
    return false;
}

int main()
{
    int n;
    cin>>n;
    for (int i=0; i<n; i++)
    {
        cin>>s;
        if (balanced())
            cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }

    return 0;
}