4/10/2017 - 3:36 PM

## 98-Validate-Binary-Search-Tree https://leetcode.com/problems/validate-binary-search-tree/

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

class Solution(object):
def isValidBST(self, root):
"""
:type root: TreeNode
:rtype: bool
"""
self.prev = None
return self.isMonotonicIncreasing(root)

def isMonotonicIncreasing(self, node):
if node == None:
return True
if not self.isMonotonicIncreasing(node.left):
return False
if self.prev != None and self.prev.val >= node.val:
return False
self.prev = node
if not self.isMonotonicIncreasing(node.right):
return False
return True``````
``````# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
def isValidBST(self, root):
"""
:type root: TreeNode
:rtype: bool
"""
return self.recursive(-float('inf'), root, float('inf'))
def recursive(self, lowBound, node, highBound):
if node == None:
return True
return node.val < highBound and node.val > lowBound and\
self.recursive(lowBound, node.left, node.val) and self.recursive(node.val, node.right, highBound)``````

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. Example 1:
``````    2
/ \
1   3
``````

Binary tree `[2,1,3]`, return true. Example 2:

``````    1
/ \
2   3
``````

Binary tree `[1,2,3]`, return false.