[leetcode tree]98. Validate Binary Search Tree

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.

先中序遍历树到数组flag,flag中必定是从小到大排列，如果不是，return false

 1 class Solution(object):
 2     def isValidBST(self, root):
 3         flag = []
 4         self.isv(root,flag)
 5         for i in range(1,len(flag)):
 6             if flag[i]<=flag[i-1]:
 7                 return False
 8         return True
 9     def isv(self,root,flag):
10         if root == None:
11             return
12         self.isv(root.left,flag)
13         flag.append(root.val)
14         self.isv(root.right,flag)