LeetCode 92.反转链表-ii

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:
    def reverseBetween(self, head: ListNode, m: int, n: int) -> ListNode:
        """
        input: 1,2,3,4,5,6  (m = 3, n = 5)
        output: 1,2,5,4,3,6
        注意：需要加pre_head 节点，以处理m=1的情况
        """
        if head is None or head.next is None:
            return head
        pre_head = ListNode(None)
        pre_head.next = head
        pre = pre_head
        for _ in range(m - 1):
            pre = pre.next        
        cur = pre.next
        for _ in range(n-m):
            tmp = cur.next
            cur.next = tmp.next
            tmp.next = pre.next
            pre.next = tmp            
        return pre_head.next 


"""
class Solution:
    def reverseBetween(self, head: ListNode, m: int, n: int) -> ListNode:
        if head is None or head.next is None:
            return head
        if m == n:
            return head
        node = ListNode(None)
        node.next = head
        left_tail = node
        cur = head 
        cnt = 1
        while cur:
            if cnt == m-1:
                left_tail = cur
            if cnt == m:
                mid_head = cur
                mid_tail = cur 
            if cnt > m:
                tmp = cur.next 
                cur.next = mid_head
                mid_head = cur
                cur = tmp
            else:
                cur = cur.next 
            if cnt == n:
                mid_tail.next = cur 
                left_tail.next = mid_head
                break
            cnt+=1
        return node.next 
"""