leetcde92 92. 反转链表 II

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反转从位置 m 到 n 的链表。请使用一趟扫描完成反转。

说明:
1 ≤ m ≤ n ≤ 链表长度。

示例:

输入: 1->2->3->4->5->NULL, m = 2, n = 4
输出: 1->4->3->2->5->NULL

class Solution {
public:
	ListNode* reverseBetween(ListNode* head, int m, int n) {
		ListNode* pre_first = 0, *next_last = 0;

		int i = 0;

		ListNode* cur = head;
		
		while (cur)
		{
			i++;
			
			if (m == i)
			{
				ListNode* pre = 0;
				ListNode* old_head = cur;

				while (i <= n)
				{
					i ++;

					ListNode* TMP = cur->next;
					cur->next = pre;

					pre = cur;
					cur = TMP;
				}
				
				/*
				反转之后，pre为反转的那段的头结点，old_head为尾结点
				*/
				if(pre_first) pre_first->next = pre;
				old_head->next = cur;

				if (pre_first == nullptr)
				{
					return pre;
				}
			}
			else
			{
				pre_first = cur;
				cur = cur->next;				
			}
		}

		return head;
	}
};