1.反转链表

题目

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL
进阶:
你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

/*function ListNode(x){
    this.val = x;
    this.next = null;
}*/
function reverseList(head){

}

code1(头插法)

function reverseList(head) {
  var prev = null
  var cur = head
  var after = null
  while (cur) {
    after = cur.next
    cur.next = prev
    prev = cur 
    cur = after
  } 
  return prev 
}

code2(递归)

function reverseList(head) {
  if (!head || !head.next)  return head
  var p = reverseList(head.next)
  head.next.next = head
  head.next = null
  return p
}