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.

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

[LeetCode]Validate Binary Search Tree