leetcode------Reverse Linked List II

标题：	Reverse Linked List II
通过率：	26.2%
难度：	中等

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 ≤ m ≤ n ≤ length of list.

单链表的逆序，遍历一遍全部逆序完成，本题不是全部逆序，是部分逆序。只用记录逆序开始点和结束点即可逆序过程如下图：

初始状态：

head->next = prev;

prev = head;

head = next;

next = head->next

第一次结束，如上图。

整个过程是一个循环的过程，如下图：

具体代码如下：

class Solution:
    def reverseBetween(self, head, m, n):
        diff, dummy, current= n - m, ListNode(0), head
        dummy.next = head
        last_unswapped = dummy
        while current != None and m > 1:
            current, last_unswapped, m = current.next, current, m - 1
        prev, first_swapped = last_unswapped, current
        while current != None and diff >= 0:
            tmp = current.next
            current.next = prev
            prev = current
            current = tmp
            diff -= 1
        last_unswapped.next, first_swapped.next = prev, current
        return dummy.next