Validate Binary Search Tree - Leetcode

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isValidBST(TreeNode root) {          Stack<TreeNode> s = new Stack<>();        ArrayList<Integer> ai = new ArrayList<>();        TreeNode pointer = root;        while(pointer!=null || !s.empty()){            if(pointer!=null){                s.push(pointer);                pointer=pointer.left;            }else{                int pre = s.peek().val;                if(ai.size()>=1 && pre<=ai.get(ai.size()-1))                   return false;                ai.add(pre);                pointer=s.pop().right;            }        }        return true;      }  }

中序遍历一棵树，看看是否有打破递增的规律。

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.

OJ's Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

   1  / \ 2   3    /   4         \     5

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".