剑指Offer24.反转链表

1. 双指针迭代

# 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:
        pre, cur = None, head
        while cur:
            tmp = cur.next
            cur.next = pre
            pre = cur
            cur = tmp
        return pre

• 时间复杂度：O(n)
• 空间复杂度：O(1)

2. 递归

# 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:
        def recur(cur, pre):
            if not cur:
                return pre
            res = recur(cur.next, cur)
            cur.next = pre
            return res
        return recur(head,None)

• 时间复杂度: O(n) 需要对链表的每个节点进行反转操作；
• 空间复杂度: O(n) 递归调用的栈空间最多为 n 层。