20 有效的括号

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class Solution:
    def isValid(self, s: str) -> bool:
        stack = ["sentry"]  # 添加哨兵
        top = 1
        for i in s:
            if stack[top-1] == "(" and i == ")":
                stack.pop()
                top -= 1
                continue
            elif stack[top-1] == "[" and i == "]":
                stack.pop()
                top -= 1
                continue
            elif stack[top-1] == "{" and i == "}":
                stack.pop()
                top -= 1
                continue
            stack.append(i)
            top += 1
            print(top, stack[top-2],stack[top-1])
        if top == 1:
            return True
        else:
            return False