올바른 괄호 [파이썬]

def solution(s):
    answer = True
    stack = []
    for str in s:
        if str == '(':
            stack.append(1)
        elif str == ')':
            if len(stack) == 0:
                return False
            else:
                stack.pop()
    if len(stack) != 0:
        return False
    return True

def solution(s):
    answer = True
    stack = []
    for a in s:
        if stack:
            if a == ')':
                stack.pop()
            else:
                stack.append(a)
        else:
            if a == ')':
                return False
            else:
                stack.append(a)
    if len(stack) != 0:
        return False
    return True