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# LeetCode98 validate-binary-search-tree（验证二叉搜索树）

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### 题目

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.

`````` 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
``````

### 方法

#### 递归法

• 时间复杂度：O(n)
• 空间复杂度：O(n)
``````class Solution {
public boolean isValidBST(TreeNode root) {
return isValidBST(root,Long.MIN_VALUE,Long.MAX_VALUE);
}
private boolean isValidBST(TreeNode root,long lower,long upper) {
if(root==null){
return true;
}
if(root.val<=lower || root.val>=upper){
return false;
}
return isValidBST(root.left, lower, root.val) && isValidBST(root.right, root.val, upper);
}
}
``````