98-Validate Binary Search Tree

100-Same Tree | Links:

题意

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

Input: [2,1,3]
Output: true

Example 2:
    5
   / \
  1   4
     / \
    3   6

Input: [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

思路

• 树：中序遍历
• 验证中序遍历后结果是升序，且没有重复元素

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """   
        inOrderSeq = self.inOrder(root)
        
        # 左 < 中 < 右
        # 没有重复元素
        return len(inOrderSeq) == len(set(inOrderSeq)) and inOrderSeq ==  sorted(inOrderSeq) 
        
    def inOrder(self, root):
        if not root: return[]
        return self.inOrder(root.left) + [root.val] +self.inOrder(root.right)

分析:

• Time: O(n)
• Space: O(n)