notes/OJ notes/pages/Leetcode Validate-Binary-Search-Tree.md

Leetcode Validate-Binary-Search-Tree

2022-07-08 10:36

Algorithms:

#algorithm #DFS #DFS_inorder

Data structures:

#DS #binary_tree #binary_search_tree

Difficulty:

#coding_problem #difficulty-medium

Additional tags:

#leetcode #CS_list_need_practicing

Revisions:

N/A

Related topics:

tag:#DFS_inorder

• Leetcode Binary-Tree-Inorder-Traversal

Links:

• Link to problem

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Problem

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid 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.

Examples

Example 1:

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

Example 2:

Input: root = [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.

Constraints

• The number of nodes in the tree is in the range [1, 104].
• -231 <= Node.val <= 231 - 1

Thoughts

[!summary] This is a #DFS #DFS_inorder problem.

I have thought a lot of recursion methods, but at last I realized:

[!tip] The feature of For a BST, DFS inorder search returns an array of ascending numbers.

The

Solution