要求：
Give a Θ(n)-time nonrecursive procedure that reverses a singly linked list of n
elements. The procedure should use no more than constant storage beyond that
needed for the list itself.
中心思想是同时取出并保存链表中的连续三个元素（previous, current, after），并翻转前两个，然后每次将这三个指针依次向后移一位并重复翻转前两个，直至结束
伪代码：
LIST-REVERSE(L)
1 previous = L.head
2 if previous.next ≠ NIL
3 current = previous.next
4 else return
5 if current.next ≠ NIL
6 after = current.next
7 else L.head = current
8 current.next = previous; previous.next = NIL
9 return
10 current.next = previous; previous.next = NIL
11 while after.next ≠ NIL
12 previous = current
13 current = after
14 after = after.next
15 current.next = previous
16 after.next = current
17 L.head = after
18 return
存储空间开销-三个指针