# 98. Validate Binary Search Tree
## Problem Statement
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`
## Intuition
```
Approach:
We Take min and max value at each level and
Modify the vals as we go left and right
For eg
5
1 6
At start -> INT_MIN, INT_MAX (for 5)
When we go left,
INT_MIN, 5
And so on
We might get a test case problem for INT_MIN , and INT_MAX itself
So taken LLONG_MIN and MAX
```
### Links
### Video Links
### Approach 1:
```
```
{% code title="C++" lineNumbers="true" %}
```cpp
class Solution {
public:
bool find_ans(TreeNode* root, long long low, long long high){
if(root == nullptr)
return true;
int value = root->val;
if(low>=value or value>=high)
return false;
if(!find_ans(root->left, low, root->val))
return false;
if(!find_ans(root->right, root->val, high))
return false;
return true;
}
bool isValidBST(TreeNode* root) {
return find_ans(root, LLONG_MIN, LLONG_MAX);
}
};
```
{% endcode %}
### Approach 2:
```
```
{% code title="C++" lineNumbers="true" %}
```cpp
```
{% endcode %}
### Approach 3:
```
```
{% code title="C++" lineNumbers="true" %}
```cpp
```
{% endcode %}
### Approach 4:
```
```
{% code title="C++" lineNumbers="true" %}
```cpp
```
{% endcode %}
### Similar Problems
###
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