括号序列

def solve(____):
    stack = []
    pairs = {'____': '____', '____': '____', '____': '____'}
    for c in s:
        if c in '____':
            stack.append(____)
        elif c in pairs:
            if not stack or stack[-____] != pairs[c]:
                print(____)
                return
            stack.pop()
    print(____)