Validate Binary Search Tree
Description:
Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a 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.
A single node tree is a BST
Example
An example:
2
/ \
1 4
/ \
3 5
The above binary tree is serialized as {2,1,4,#,#,3,5} (in level order).
Code:
"""
Definition of TreeNode:
class TreeNode:
def __init__(self, val):
self.val = val
self.left, self.right = None, None
"""
class Solution:
"""
@param root: The root of binary tree.
@return: True if the binary tree is BST, or false
"""
def isValidBST(self, root):
# write your code here
if root and not root.left and not root.right:
return True
cur = [-2147483649]
res = [True]
def travel(root):
if not root:
return
travel(root.left)
print(cur[0])
if root.val > cur[0]:
cur[0] = root.val
else:
res[0] = False
travel(root.right)
travel(root)
return res[0]