Leetcode-Validate BST

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

WRONG solution:

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public boolean isValidBST(TreeNode root) {
12         if (root==null)
13             return true;
14
15         return isValidBSTRecur(root);
16     }
17
18     public boolean isValidBSTRecur(TreeNode curNode){
19         if (curNode.left==null&&curNode.right==null)
20             return true;
21
22         boolean leftValid=true;
23         boolean rightValid=true;
24
25         if (curNode.left!=null){
26             if (curNode.left.val>=curNode.val)
27                 return false;
28             leftValid = isValidBSTRecur(curNode.left);
29         }
30
31         if (curNode.right!=null){
32             if (curNode.right.val<=curNode.val)
33                 return false;
34             rightValid = isValidBSTRecur(curNode.right);
35         }
36
37         if (leftValid&&rightValid)
38             return true;
39         else
40             return false;
41     }
42 }

This is wrong! Because we only consider about one level up! Example:

Correct Solution:

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public boolean isValidBST(TreeNode root) {
12         if (root==null)
13             return true;
14
15         List<Integer> queue = new ArrayList<Integer>();
16         isValidBSTRecur(root,queue);
17
18         for (int i=1;i<queue.size();i++)
19             if (queue.get(i)<=queue.get(i-1))
20                 return false;
21
22         return true;
23     }
24
25     public void isValidBSTRecur(TreeNode curNode, List<Integer> queue){
26         if (curNode.left==null&&curNode.right==null){
27             queue.add(curNode.val);
28             return;
29         }
30
31
32         if (curNode.left!=null)
33             isValidBSTRecur(curNode.left,queue);
34         queue.add(curNode.val);
35         if (curNode.right!=null)
36             isValidBSTRecur(curNode.right,queue);
37
38         return;
39     }
40 }

We put nodes into a list in the preorder, then check whether the list is monotonic increasing.