Valid BST

TreeNode.java

package com.interviewbytes.trees;

public class TreeNode {

    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

ValidBST.java

package com.interviewbytes.trees;

public class ValidBST {

    public boolean isValidBST(TreeNode root) {
        return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    private boolean helper(TreeNode root, long min, long max) {

        return root == null
                || root.val > min && root.val < max
                && helper(root.left, min, root.val)
                && helper(root.right, root.val, max);
    }
}