206. Reverse Linked List(javascr

Reverse a singly linked list.
反转链表。

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

Follow up:
A linked list can be reversed either iteratively or recursively. Could you implement both?
用迭代和递归两种方法实现

迭代：

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) { 
  if (head === null || head.next === null) {    // 链表为空或只有一个节点时，不用反转
    return head;
  }
  var p = head.next;
  head.next = null;    // 让原本的head变为尾节点
  var temp;    // 临时指针
  while (p !== null) {
    temp = p.next;
    p.next = head;
    head = p;
    p = temp;
  }
  return head
    
};

递归：

var reverseList = function(head) {
   if (head === null || head.next === null) {
    return head;
  }
  
  var new_head = reverseList(head.next);  // 反转后的头节点
  head.next.next = head;                  // 将反转后的链表的尾节点与当前节点相连
  head.next = null;
  return new_head  
};