LeetCode - Recover Binary Search Tree

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?

Solution:

(using O(n) space)

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    ArrayList<TreeNode> nodes;
    ArrayList<Integer> values;
    public void inOrder(TreeNode root){
        if(root == null) return;
        
        inOrder(root.left);
        
        values.add(root.val);
        nodes.add(root);
        
        inOrder(root.right);
    }
    public void recoverTree(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        nodes = new ArrayList<TreeNode>();
        values = new ArrayList<Integer>();
        inOrder(root);
        Collections.sort(values);
        int index = 0;
        for(TreeNode n : nodes){
            n.val = values.get(index++);
        }
        
    }
}