LeetCode OJ 98. 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.

判断一棵二叉树是否是二叉搜索树，我们可以采用中序遍历来看二叉树的中序遍历是否是递增的，如果是递增的，那么就是BST，如果不是递增的，那么就不是BST。

代码如下：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public long n = (long)Integer.MIN_VALUE-1;
    public boolean isValidBST(TreeNode root) {
        if(root == null) return true;
        boolean a = isValidBST(root.left);
        if(root.val<=n) return false;
        else n = root.val;
        boolean b = isValidBST(root.right);
        return (a&&b);
    }
}