It would be an easy problem if Binary Tree Level Order Traversal works. Yet, a queue is not allow because of that constant space is required.
connecting left and right child is easy, just node.left.next = node.right
Before
1
/ \
2 3
/ \ / \
4 5 6 7
After
1
/ \
2 -> 3
/ \ / \
4-> 5 6->7
5 and 6 shoud be connected。
when connecting 4 and 5, 2 and 3 is already connected, that is a good news, we can connect 5 and 6 using 2.next.
node.right.next = node.next.left;