Recover Binary Search Tree Solutions in Java

/// Source : https://leetcode.com/problems/validate-binary-search-tree/description/
/// Author : liuyubobobo
/// Time   : 2018-05-22

import java.util.ArrayList;

/// Using inOrder traverse
/// Store all Nodes in an ArrayList and find the mistaked swapped nodes
///
/// Time Complexity: O(n)
/// Space Complexity: O(n)
class Solution {

    public void recoverTree(TreeNode root) {

        ArrayList<TreeNode> list = new ArrayList<>();
        inOrder(root, list);

        TreeNode a = null;
        TreeNode b = null;
        for(int i = 1 ; i < list.size() ; i ++) {
            if (list.get(i - 1).val > list.get(i).val) {
                if (a == null){
                    a = list.get(i - 1);
                    b = list.get(i);
                }
                else {
                    b = list.get(i);
                    break;
                }
            }
        }

        int t = a.val;
        a.val = b.val;
        b.val = t;
    }

    private void inOrder(TreeNode node, ArrayList<TreeNode> list){

        if(node == null)
            return;

        inOrder(node.left, list);
        list.add(node);
        inOrder(node.right, list);
    }

    public static void main(String[] args){

        TreeNode root = new TreeNode(3);
        root.left = new TreeNode(1);
        root.right = new TreeNode(4);
        root.right.left = new TreeNode(2);

        (new Solution()).recoverTree(root);
    }
}

/// Source : https://leetcode.com/problems/validate-binary-search-tree/description/
/// Author : liuyubobobo
/// Time   : 2018-05-22

/// Using inOrder traverse
/// Find the mistaked swapped nodes during the traversing
///
/// Time Complexity: O(n)
/// Space Complexity: O(h)
class Solution2 {

    private TreeNode pre;
    private TreeNode a, b;

    public void recoverTree(TreeNode root) {

        pre = null;
        a = null;
        b = null;
        inOrder(root);

        int t = a.val;
        a.val = b.val;
        b.val = t;
    }

    private void inOrder(TreeNode node){

        if(node == null)
            return;

        inOrder(node.left);

        if(pre != null && pre.val > node.val){
            if (a == null){
                a = pre;
                b = node;
            }
            else
                b = node;
        }
        pre = node;

        inOrder(node.right);
    }
}

/// Source : https://leetcode.com/problems/validate-binary-search-tree/description/
/// Author : liuyubobobo
/// Time   : 2018-05-29

/// Using Morris inOrder traversal
/// Find the mistaked swapped nodes during the traversing
///
/// Time Complexity: O(n)
/// Space Complexity: O(1)
class Solution3 {

    private TreeNode pre;
    private TreeNode a, b;

    public void recoverTree(TreeNode root) {

        TreeNode cur = root;
        while(cur != null){
            if(cur.left == null){
                process(cur);
                cur = cur.right;
            }
            else{
                TreeNode prev = cur.left;
                while(prev.right != null && prev.right != cur)
                    prev = prev.right;

                if(prev.right == null){
                    prev.right = cur;
                    cur = cur.left;
                }
                else{
                    prev.right = null;
                    process(cur);
                    cur = cur.right;
                }
            }
        }

        int t = a.val;
        a.val = b.val;
        b.val = t;
    }

    private void process(TreeNode node){

        if(pre != null && pre.val > node.val){
            if(a == null){
                a = pre;
                b = node;
            }
            else
                b = node;
        }

        pre = node;
    }
}