[leetcode] Validate Binary Search Tree （检验是否为二叉查找树） Python

class Solution:
    # @param root, a tree node
    # @return a boolean
    def isValidBST(self, root):
        if root is None:
            return True
        return self.helper(root,-99999,999999)
    
    # left ~ right 是node的取值范围
    def helper(self,root,left,right):
        
        if root is None:
            return True
        if root.val <= left or root.val >=right:
            #超出范围
            return False
        return self.helper(root.left,left,root.val) and self.helper(root.right,root.val,right)