validate-binary-search-tree

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Algorithms

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

A valid BST is defined as follows:

<ul>
	<li>The left subtree of a node contains only nodes with keys less than the node&#39;s key.</li>
	<li>The right subtree of a node contains only nodes with keys greater than the node&#39;s key.</li>
	<li>Both the left and right subtrees must also be binary search trees.</li>
</ul>

&nbsp;
Example 1:
<img alt="" src="https://assets.leetcode.com/uploads/2020/12/01/tree1.jpg" style="width: 302px; height: 182px;" />
<pre>
Input: root = [2,1,3]
Output: true
</pre>

Example 2:
<img alt="" src="https://assets.leetcode.com/uploads/2020/12/01/tree2.jpg" style="width: 422px; height: 292px;" />
<pre>
Input: root = [5,1,4,null,null,3,6]
Output: false
Explanation: The root node&#39;s value is 5 but its right child&#39;s value is 4.
</pre>

&nbsp;
Constraints:

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 104]</code>.</li>
	<li><code>-231 &lt;= Node.val &lt;= 231 - 1</code></li>
</ul>

isValidBST

Validate Binary Search Tree

Binary Tree Inorder Traversal

binary-tree-inorder-traversal

二叉树的中序遍历

Find Mode in Binary Search Tree

find-mode-in-binary-search-tree

二叉搜索树中的众数

Tree

Depth-First Search

Binary Search Tree

Binary Tree

给你一个二叉树的根节点 <code>root</code> ，判断其是否是一个有效的二叉搜索树。

有效 二叉搜索树定义如下：

<ul>
	<li>节点的左子树只包含 小于 当前节点的数。</li>
	<li>节点的右子树只包含 大于 当前节点的数。</li>
	<li>所有左子树和右子树自身必须也是二叉搜索树。</li>
</ul>

&nbsp;

示例 1：
<img alt="" src="https://assets.leetcode.com/uploads/2020/12/01/tree1.jpg" style="width: 302px; height: 182px;" />
<pre>
输入：root = [2,1,3]
输出：true
</pre>

示例 2：
<img alt="" src="https://assets.leetcode.com/uploads/2020/12/01/tree2.jpg" style="width: 422px; height: 292px;" />
<pre>
输入：root = [5,1,4,null,null,3,6]
输出：false
解释：根节点的值是 5 ，但是右子节点的值是 4 。
</pre>

&nbsp;

提示：

<ul>
	<li>树中节点数目范围在<code>[1, 104]</code> 内</li>
	<li><code>-231 &lt;= Node.val &lt;= 231 - 1</code></li>
</ul>