LeetCode - 206. Reverse Linked List

Reverse a singly linked list.

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?

反转单链表，看到我好几年前通过的代码居然是用List遍历保存之后再倒序遍历反转，然而还RE了好几次。。。往事突然浮现在脑海，没忍住又重写了一下。

耗时0ms

class Solution {
public ListNode reverseList(ListNode head) {
ListNode cur = head, prev = null, temp = null;
while (cur != null) {
temp = cur.next;
cur.next = prev;
prev = cur;
cur = temp;
}
return prev;
}
}

以前的代码是这样的（耗时464ms）：

public class Solution {
public ListNode reverseList(ListNode head) {
if(head == null || head.next == null)
return head;
ArrayList<Integer> list = new ArrayList<Integer>();
ListNode p = head;
while(p!=null) {
list.add(p.val);
p = p.next;
}
ListNode q = head;
for(int i=list.size()-1; i>=0; i--) {
q.val = list.get(i);
q = q.next;
}
return head;

}
}