[레벨 2] 올바른 괄호

#include <string>
#include <iostream>
#include <stack>
using namespace std;

bool solution(string s)
{
    bool answer = true;
    stack<char> st;
    int len = s.size();
    if (s[0] == ')')
        return false;
    for (int i = 0; i < len; ++i)
    {
        if (s[i] == ')')
        {
            if (!st.empty() && st.top() == '(')
                st.pop();
            else
                st.push(s[i]);
        }
        else
            st.push(s[i]);
    }
    if (!st.empty())
        answer = false;
    return answer;
}