LeetCode 98. 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.

Example 1:

Input:
    2
   / \
  1   3
Output: true

Example 2:

    5
   / \
  1   4
     / \
    3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value
             is 5 but its right child's value is 4.

	/**
	* Definition for a binary tree node.
	* public class TreeNode {
	* int val;
	* TreeNode left;
	* TreeNode right;
	* TreeNode(int x) { val = x; }
	* }
	*/
	class Solution {
	public boolean isValidBST(TreeNode root) {
	// corner cases
	if (root == null) {
	return true;
	}
	return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
	}
	public boolean isValidBST(TreeNode root, long min, long max) {
	// corner cases
	if (root == null) {
	return true;
	}
	if (root.val <= min || root.val >= max) {
	return false;
	}
	return isValidBST(root.left, min, Math.min(max, root.val)) && isValidBST(root.right, Math.max(min, root.val), max);
	}
	}

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