ValidateBinarySearchTree98.kt

package medium
/**
 * 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
 */
fun isValidBST(root: TreeNode?): Boolean {
    return valid(root, Long.MIN_VALUE, Long.MAX_VALUE)
}
fun valid(root: TreeNode?,start:Long,end:Long):Boolean
{
    if(root==null) return true

    if(root.`val` in (start + 1) until end)
    {
        return  valid(root.left,start,root.`val`.toLong()) &&
                valid(root.right,root.`val`.toLong(),end)
    }
    return false
}