题:https://leetcode.com/problems/validate-binary-search-tree/description/](https://leetc
ode.com/problems/validate-binary-search-tree/description/)
题目
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:
Input:
2
/ \
1 3
Output: true
Example 2:
5
/ \
1 4
/ \
3 6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node’s value
is 5 but its right child’s value is 4.
思路
这里用到了一个性质,对于 Binary Search Tree ,它的中序遍历是严格单调递增。
另外, 可能使用用动态规划的想法,只要 左树的max < 当前节点 < 右树的min,具体的实现还要进一步思考。
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):
"""
:type root: TreeNode
:rtype: bool
"""
tstack = []
node = root
tmp = None
while node != None or len(tstack) != 0:
while node != None:
tstack.append(node)
node = node.left
node = tstack.pop(-1)
print(node.val)
if tmp != None:
if not tmp<node.val:
return False
tmp = node.val
node = node.right
return True