Recover Binary Search Tree

1. Title

2. Http address

https://leetcode.com/problems/recover-binary-search-tree/

3. The question

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.

Note:
A solution using O(n) space is pretty straight forward. Could you devise a constant space solution?

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

4. My code (AC)

 1     void recoverTreeHelp(TreeNode root,TreeNode input []) {
 2         
 3         if ( root == null)
 4             return;
 5         
 6         recoverTreeHelp(root.left, input);
 7         
 8         if ( input[2] != null)
 9         {
10             if ( input[2].val > root.val){
11                 input[1] = root;
12                 input[0] = input[0] != null ? input[0] : input[2];
13             }    
14         }
15         
16         input[2] = root;
17         
18         recoverTreeHelp(root.right,input);
19         return;
20     }
21     
22     public void recoverTree(TreeNode root) {
23         
24         TreeNode input [] = new TreeNode[3];
25         recoverTreeHelp(root, input);
26         if( input[0] != null && input[1] != null)
27         {
28             int tmp = input[0].val;
29             input[0].val = input[1].val;
30             input[1].val = tmp;
31         }
32     }