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:

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.

# 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
        """
        if not root:
            return True
        list = []
        def inorder(root):
            if root.left:
                inorder(root.left)
            list.append(root.val)
            if root.right:
                inorder(root.right)
        inorder(root)
        for i in range(len(list)-1):
            if list[i]>=list[i+1]:
                return False
        return True