BST-----Validate Binary Search Tree

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.
Have you met this question in a real interview? Yes
Example
An example:

2
/ \
1 4
/ \
3 5
The above binary tree is serialized as {2,1,4,#,#,3,5} (in level order).

递归

/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */public class Solution {    /**     * @param root: The root of binary tree.     * @return: True if the binary tree is BST, or false     */    public boolean isValidBST(TreeNode root) {        // write your code here        if(root == null) return true;        return helper(root , Long.MIN_VALUE, Long.MAX_VALUE);    }    private boolean helper (TreeNode root , long lower, long upper ){        if(root ==null)return true;        if(root.val>=upper||root.val<=lower)return false;        boolean isLeftValidBST= helper(root.left, lower, root.val);        boolean isRightValidBST =helper(root.right, root.val, upper);        return isLeftValidBST&&isRightValidBST;    }}