leetcode--Validate Binary Search Tree

问题描述：

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.

采用中序遍历，如果遍历过程严格服从从小到大，则在最后返回True，否则返回False

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return a boolean
    def isValidBST(self, root):
        def inOrder(root):
            stack = []
            p = root
            minVal = None
            while p or len(stack) > 0:
                if p:
                    stack.append(p)
                    p = p.left
                else:
                    p = stack.pop()
                    if None == minVal or minVal < p.val:
                        minVal = p.val
                        p = p.right
                    else:
                        return False
            return True
        return inOrder(root)