LeetCode

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;
        
    }
}