通过检查是否存在循环来确定二叉树有效性的函数

class Solution:
    def isValidBST(self, root):
        def isValid(node, lower, upper):
            if not node:
                return True
            if node.val <= lower or node.val >= upper:
                return False
            return isValid(node.left, lower, node.val) and isValid(node.right, node.val, upper)
        
        return isValid(root, float('-inf'), float('inf'))

from collections import deque

class Solution:
    def isValidBST(self, root):
        if not root:
            return True
        
        queue = deque([(root, float('-inf'), float('inf'))])
        while queue:
            node, lower, upper = queue.popleft()
            if not node:
                continue
            if node.val <= lower or node.val >= upper:
                return False
            queue.append((node.left, lower, node.val-1))
            queue.append((node.right, node.val+1, upper))
        
        return True