[leetcode][98] Validate Binary Search Tree

没睡着，来做题。

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.

# 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 getMaxVal(self, node):
        while node.right:
            node = node.right
        return node.val
        
    def getMinVal(self,node):
        while node.left:
            node = node.left
        return node.val
            
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if root is None:
            return True
        if (root.left is not None) and (root.right is not None):
            # max Val in left subtree and min val in right subtree
            maxVal = self.getMaxVal(root.left)
            minVal = self.getMinVal(root.right)
            if root.val > maxVal and root.val < minVal:
                return self.isValidBST(root.left) and self.isValidBST(root.right)
            else:
                return False
        elif root.left is not None:
            # max val in left subtree
            maxVal = self.getMaxVal(root.left)
            if maxVal < root.val:
                return self.isValidBST(root.left)
            else:
                return False
        elif root.right is not None:
            # min val in right subtree
            minVal = self.getMinVal(root.right)
            if minVal > root.val:
                return self.isValidBST(root.right)
            else:
                return False
        else:
            return True