IsBST - is binary search tree

gistfile1.java

//  Definition for binary tree
class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;
 
	TreeNode(int x) {
		val = x;
	}
}
 
public class Solution {
 
	public static boolean isValidBST(TreeNode root) {
		return validate(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
	}
 
	public static boolean validate(TreeNode root, int min, int max) {
		if (root == null) {
			return true;
		}
 
		// not in range
		if (root.val <= min || root.val >= max) {
			return false;
		}
 
		// left subtree must be < root.val && right subtree must be > root.val
		return validate(root.left, min, root.val) && validate(root.right, root.val, max);
	}
}

With this, you can't ever possibly have Integer.MIN_VALUE or Integer.MAX_VALUE as values, even on the left and right side of your tree.
A BST requires that all nodes are in ascending order from left to right: adding an arbitrary limit has that limitation in a number situation, but it's even more annoying with strings.
Above and beyond that, you are never changing the value of what min and max are: this won't even work when the min and max are within the limits of an int.