No.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：[]

提示：

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

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？

解题思路

实现

迭代

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
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 prev = null;
  let current = head;
  while (current) {
    let next = current.next;
    current.next = prev;
    prev = current;
    current = next;
  }
  return prev;
};

递归

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
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) {
  if (!head || !head.next) return head;
  let node = reverseList(head.next);
  head.next.next = head;
  head.next = null;
  return node;
};