Recover Binary Search Tree

Source

• 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?

Java O(longN)

public class Solution {

    TreeNode firstElement = null;
    TreeNode secondElement = null;
    // The reason for this initialization is to avoid null pointer exception in the first comparison when prevElement has not been initialized
    TreeNode prevElement = new TreeNode(Integer.MIN_VALUE);

    public void recoverTree(TreeNode root) {

        // In order traversal to find the two elements
        traverse(root);

        // Swap the values of the two nodes
        int temp = firstElement.val;
        firstElement.val = secondElement.val;
        secondElement.val = temp;
    }

    private void traverse(TreeNode root) {

        if (root == null)
            return;

        traverse(root.left);

        // Start of "do some business", 
        // If first element has not been found, assign it to prevElement (refer to 6 in the example above)
        if (firstElement == null && prevElement.val >= root.val) {
            firstElement = prevElement;
        }

        // If first element is found, assign the second element to the root (refer to 2 in the example above)
        if (firstElement != null && prevElement.val >= root.val) {
            secondElement = root;
        }        
        prevElement = root;

        // End of "do some business"

        traverse(root.right);
    }
}

Java O(1) Morris Traversal

Reference

Detail Explain about How Morris Traversal Finds two Incorrect Pointer