206. 反转链表

题目来源：206. 反转链表

给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。

 

示例 1：

输入：head = [1,2,3,4,5]
输出：[5,4,3,2,1]

示例 2：

输入：head = [1,2]
输出：[2,1]

示例 3：

输入：head = []
输出：[]

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
    let cur = head;
    let pre = null;
    while(cur != null){
        let tmp = cur.next;
        cur.next = pre;
        pre = cur;
        cur = tmp;
    }
    return pre;
};

提示：

• 链表中节点的数目范围是 [0, 5000]
• -5000 <= Node.val <= 5000

 Python3

# 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:
      pre, cur = None, head
      while cur:
        nxt = cur.next
        cur.next = pre
        pre = cur
        cur = nxt

      return pre

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
      if not head or not head.next:
        return head

      newHead = self.reverseList(head.next)
      head.next.next = head
      head.next = None

      return newHead      

 

进阶：链表可以选用迭代或递归方式完成反转。你能否用两种方法解决这道题？