[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

 class Solution(object):
     def isValidBST(self, root):
         flag = []
         self.isv(root,flag)
         for i in range(1,len(flag)):
             if flag[i]<=flag[i-1]:
                 return False
         return True
     def isv(self,root,flag):
         if root == None:
             return
         self.isv(root.left,flag)
         flag.append(root.val)
         self.isv(root.right,flag)