Validate Binary Search Tree

题目：

Given a binary tree, determine if it is a valid Binary Search Tree (BST)

解答：

public class Solution {

    public boolean isValidBST(TreeNode root) {
        return valid(root, null, null);
    }

    private boolean valid(TreeNode p, Integer low, Integer high) {
        if(p == null) {
            return true;
        }

        return (low == null || p.val > low) && (high == null || p.val < high)
                && valid(p.left, low, p.val) && valid(p.right, p.val, high);
    }   

}