综合：有效括号（栈应用）

def solve():
    def is_valid(____):
        stack = []
        mapping = {'____': '____', '____': '____', '____': '____'}
        for ch in s:
            if ch in '____':
                stack.append(____)
            elif not stack or stack[-____] != mapping[ch]:
                return False
            else:
                stack.pop()
        return len(____) == ____
    print(is_valid(____))
    print(is_valid(____))