题目描述

Reverse Linked List

反转一个链表

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

反转链表一般有迭代法和递归法，能不能使用两种解法都实现

From leetcode No.206 Easy (opens new window)

/**
 * 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) {
    
};

解法

点击查看

/**
 * 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 curr = head
    let prev = null
    while(curr){
        prev = new ListNode(curr.val, prev)
        curr = curr.next
    }
    return prev
};

总结

iteratively

var reverseList = function(head) {
    let curr = head
    let prev = null
    let next = null
    while(curr){
      next = curr.next
      curr.next = prev
      prev = curr
      curr = next
    }
    return prev
};

recursively

var reverseList = function(head) {
  return _reverse(head, null)
};

function _reverse(curr, prev) {
  if (curr == null) return prev
  
  const next = curr.next
  curr.next = prev
  return _reverse(next, curr)
}