LeetCode-206. Reverse Linked List

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问题描述

Given the head of a singly linked list, reverse the list, and return the reversed list.

Example:

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Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Constraints:

  • The number of nodes in the list is the range [0, 5000].
  • -5000 <= Node.val <= 5000

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

解答

解法一：迭代
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/*
 * @Description: 反转链表
 * @Author: libk
 * @Github: https://github.com/libk
 */
/**
 * 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}
 */
const reverseList = function(head) {
  let pre = null
  let cur = head
  while (cur !== null) {
    let temp = cur.next
    cur.next = pre
    pre = cur
    cur = temp
  }
  return pre
}
解法二：递归
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/*
 * @Description: 反转链表
 * @Author: libk
 * @Github: https://github.com/libk
 */
/**
 * 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}
 */
const reverseList = function(head) {
  if (head === null || head.next === null) {
    return head
  }
  let newHead = reverseList(head.next)
  head.next.next = head
  head.next = null
  return newHead
}