動機

當初還想著怎麼看node…

Problem

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid BST is defined as follows:

  • The left subtree of a node contains only nodes with keys less than the node's key.
  • The right subtree of a node contains only nodes with keys greater than the node's key.
  • Both the left and right subtrees must also be binary search trees.

 

Example 1:

Input: root = [2,1,3]Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]Output: falseExplanation: The root node's value is 5 but its right child's value is 4.

 

Constraints:

  • The number of nodes in the tree is in the range [1, 104].
  • -231 <= Node.val <= 231 - 1

Sol

直接把範圍傳下去就好了

class Solution:
    def isValidBST(self, root: TreeNode,l=float('-inf'),r=float('inf')) -> bool:
        if not root:
            return True
        elif root.val <= l or r <= root.val:
            return False
        else:
            return self.isValidBST(root.left, l, root.val) and self.isValidBST(root.right, root.val, r)