98.Validate Binary Search Tree

Description

	#98	Validate Binary Search Tree	Medium
		Tree	MS
Given the root of a binary tree, determine if it is a valid binary search tree (BST).

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.

Recursive

class Solution {
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, null, null);
    }

    private boolean isValidBST(TreeNode root, Integer lessThan, Integer greaterThan) {
        if(root == null) return true;
        if(lessThan != null && root.val >= lessThan) return false;
        if(greaterThan != null && root.val <= greaterThan) return false;
        if(root.left != null && root.left.val >= root.val) return false;
        if(root.right != null && root.right.val <= root.val) return false;
        return isValidBST(root.left, root.val, greaterThan) && isValidBST(root.right, lessThan, root.val);
    }
}