打卡群2刷题总结1007——反转链表

示例:
输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

进阶:
你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        # head为空
        if not head:
            return head
        
        # p、q、r为相邻三个节点
        # q.next指向p，同时三个指针后移
        p, q, r = head, head.next, head
        while q:
            r = q.next
            q.next = p
            p = q
            q = r
        
        # 修改head.next及head
        head.next = None
        head = p
        return p