【leetcode系列】【算法】【中等】验证二叉搜索树

题目：

题目链接： https://leetcode-cn.com/problems/validate-binary-search-tree/

解题思路：

从底向上的判断每个节点的左右子树和自己的值大小，如果不符合二叉搜索数的要求，返回false，结束所有递归；如果符合要求，将当前的值列表返回给上一层，继续递归

代码实现：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        def valid_help(root, path):
            if not root:
                return True
            
            left = []
            if root.left and False == valid_help(root.left, left):
                return False
                
            right = []
            if root.right and False == valid_help(root.right, right):
                return False
                
            if 0 < len(left) and max(left) >= root.val:
                return False
            elif 0 < len(right) and min(right) <= root.val:
                return False
            
            path += left + right + [root.val]
            return True
            
        return valid_help(root, [])