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

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+# 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:
+```expander
+tag:#DFS_inorder
+```
+ 
+- [[Leetcode Binary-Tree-Inorder-Traversal]]
+ 
+
+##### Links:
+- [Link to problem](https://leetcode.com/problems/validate-binary-search-tree/)
+___
+### 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:**
+
+![](https://assets.leetcode.com/uploads/2020/12/01/tree1.jpg)
+
+**Input:** root = [2,1,3]
+**Output:** true
+
+**Example 2:**
+
+![](https://assets.leetcode.com/uploads/2020/12/01/tree2.jpg)
+
+**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