# 验证二叉搜索树

<div class="notranslate">
    <p>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。</p>

    <p><strong>有效</strong> 二叉搜索树定义如下：</p>

    <ul>
        <li>节点的左子树只包含<strong> 小于 </strong>当前节点的数。</li>
        <li>节点的右子树只包含 <strong>大于</strong> 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>

    <p>&nbsp;</p>

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

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

    <p>&nbsp;</p>

    <p><strong>提示：</strong></p>

    <ul>
        <li>树中节点数目范围在<code>[1, 10<sup>4</sup>]</code> 内</li>
        <li><code>-2<sup>31</sup> &lt;= Node.val &lt;= 2<sup>31</sup> - 1</code></li>
    </ul>
</div>

## template

```java

public class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;

	TreeNode(int x) {
		val = x;
	}
}

class Solution {
	public boolean isValidBST(TreeNode root) {

		if (root == null)
			return true;
		if (root.left == null && root.right == null) {
			return true;
		}
		if (root.left != null) {
			TreeNode cur = root.left;
			while (cur.right != null) {
				cur = cur.right;
			}
			if (cur.val >= root.val) {
				return false;
			}
		}
		if (root.right != null) {
			TreeNode cur = root.right;
			while (cur.left != null) {
				cur = cur.left;
			}
			if (cur.val <= root.val) {
				return false;
			}
		}

		boolean left = isValidBST(root.left);
		boolean right = isValidBST(root.right);

		return left && right;
	}
}
```

## 答案

```java

```

## 选项

### A

```java

```

### B

```java

```

### C

```java

```