Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,
return 1->4->3->2->5->NULL.

Note: Given m, n satisfy the following condition:

1. 1 ≤ m ≤ n ≤ length of list.
2. head 的序号为 1

见程序注释