LeetCode | 0098. Validate Binary Search Tree驗證二叉搜索樹【Python】

LeetCode 0098. Validate Binary Search Tree驗證二叉搜索樹【Medium】【Python】【二叉樹】

Problem

LeetCode

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.

問題

力扣

給定一個二叉樹，判斷其是否是一個有效的二叉搜索樹。

假設一個二叉搜索樹具有如下特徵：

• 節點的左子樹只包含小於當前節點的數。
• 節點的右子樹只包含大於當前節點的數。
• 所有左子樹和右子樹自身必須也是二叉搜索樹。

示例 1:

輸入:
    2
   / \
  1   3
輸出: true

示例 2:

輸入:
    5
   / \
  1   4
     / \
    3   6
輸出: false
解釋: 輸入爲: [5,1,4,null,null,3,6]。
     根節點的值爲 5 ，但是其右子節點值爲 4 。

思路

二叉樹

root 不只需要和左右子節點比較，而是和整個左右子樹所有節點比較。

Python3代碼

# 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: TreeNode) -> bool:
        return self.isValid(root, None, None)
    
    def isValid(self, root: TreeNode, min_: TreeNode, max_: TreeNode):
        if not root:
            return True
        if min_ != None and root.val <= min_.val:
            return False
        if max_ != None and root.val >= max_.val:
            return False
        return self.isValid(root.left, min_, root) and self.isValid(root.right, root, max_)

GitHub鏈接

Python