# Validate Binary Search Tree

> Given a binary tree, determine if it is a valid binary search tree (BST).

> Assume a 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.

这题需要判断是不是一个正确的二叉搜索树,比较简单地一道题。

我们通过递归整棵树来解决,代码如下:

```c++
class Solution {
public:
bool isValidBST(TreeNode *root) {
return valid(root, numeric_limits::min(), numeric_limits::max());
}

bool valid(TreeNode* node, int minVal, int maxVal) {
if(!node) {
return true;
}

if(node->val <= minVal || node->val >= maxVal) {
return false;
}

return valid(node->left, minVal, node->val) &&
valid(node->right, node->val, maxVal);
}
};
```