Validate BST

ValidateBST.java

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