[leetcode] 98. Validate Binary Search Tree @ python

版权声明：版权归个人所有，未经博主允许，禁止转载 https://blog.csdn.net/danspace1/article/details/86679552

原题

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:

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.

解法

中序遍历, 将节点的值放入列表, 然后检查列表是否为递增序列, 根据题意, BST中序遍历后一定是递增序列, 如果不是则返回False.
Time: 2*O(n)
Space: O(n)

代码

# 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 isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if not root:
            return True
        
        l = []
        def inOrder(root):
            if not root:
                return
            inOrder(root.left)
            l.append(root.val)
            inOrder(root.right)
            
        inOrder(root)
        for i in range(1, len(l)):
            if l[i] <= l[i-1]:
                return False
            
        return True