LeetCode_Reverse Linked List

Reverse Linked List

Reverse a singly linked list.
（翻转链表）

Note:
A linked list can be reversed either iteratively or recursively. Could you implement both?

Example:

1. 迭代法

构建一个头结点，在遍历链表的过程中不断地采用头插法即可翻转链表。具体实现过程如下：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        new_head = ListNode(0)
        
        p = head
        while p:
            tmp = p.next
            p.next = new_head.next
            new_head.next = p
            p = tmp
        
        return new_head.next

2. 递归法

递归的翻转后面的字符串，然后将当前的头结点放在最尾部，具体实现过程如下：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        if not head or not head.next:
            return head
        
        new_head = self.reverseList(head.next)
        head.next.next = head
        head.next = None
        
        return new_head