/***
 * 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 {
    // Solution O(n), O(log n)
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    private boolean isValidBST(TreeNode root, long lower, long upper) {
        if (root == null) return true;
        return ((long)root.val > lower) && ((long)root.val < upper) &&
            isValidBST(root.left, lower, root.val) &&
            isValidBST(root.right, root.val, upper);
    }
}

add ValidateBinarySearchTree.java