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.

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

 

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 * 详细解释
 * http://blog.csdn.net/sgbfblog/article/details/7771096
 */
public class Solution {
    public boolean isValidBSTHelper(TreeNode root, int min, int max) {
        if (root==null) {
            return true;
        }
        return root.val > min && root.val < max && isValidBSTHelper(root.left, min, root.val) && isValidBSTHelper(root.right, root.val, max);
    }
    public boolean isValidBST(TreeNode root) {
        return isValidBSTHelper(root,Integer.MIN_VALUE, Integer.MAX_VALUE);
    }
}