[Leetcode Series] [] [medium] verification algorithm binary search tree

topic:

Topic links:  https://leetcode-cn.com/problems/validate-binary-search-tree/

 

Problem-solving ideas:

Up from the bottom left sub-tree and each node determines its own value of the size, if the number does not meet the requirements of the binary search, it returns false, all recursion ends; if they meet the requirements, the current value is returned to the list one level, recursively

 

Code:

# 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, [])