Implement an inorder traversal with o(1) space

InOrderTraversalWithConstantSpace.java

import java.util.ArrayList;
import java.util.List;

/*
  Problem: Perform in order traversal of binary tree without using recursion or stack (No space)
*/
public class InOrderTraversalWithConstantSpace {

    public void inOrderTraversal(BinaryTree<Integer> tree) {
        BinaryTree<Integer> prev = null;
        BinaryTree<Integer> current = tree;
        List<Integer> result = new ArrayList<>();
        while (current != null) {
            BinaryTree<Integer> next;
            // If previous node equal to parent means we are in the left tree
            if (current.parent == prev) {
                //Start iterating through the left side tree
                if (current.left != null) {
                    next = current.left;
                } else {
                    result.add(current.data);
                    next = (current.right != null ? current : current.parent);
                }
            } else if (prev == current.left) {
                // If previous is current.left that means we just finished going through left node
                // so add current data to list and start moving in right direction
                result.add(current.data);
                next = (current.right != null ? current : current.parent);
            } else {
                //Done with both right and left sibling so move up
                next = current.parent;
            }
            prev = current;
            current = next;
        }
    }
}