eg. 5

1 -> ? -> 5
2 -> ? -> 4
3 -> ? -> 3
4 -> ? -> 2
5 -> ? -> 1

Case1:

1 -> ? -> 5 -> ? -> 1 2 -> ? -> 4 -> ? -> 2

merge above

1 -> 2 -> 5 -> 4 -> 1

Case2:

3 -> 3 -> 3