Leetcode: Reverse 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?

基本迭代

Time: O(N), Space: O(1)

 1 class Solution {
 2     public ListNode reverseList(ListNode head) {
 3         if (head == null) return null;
 4         ListNode p1 = head;
 5         ListNode p2 = head.next;
 6         while (p2 != null) {
 7             ListNode next = p2.next;
 8             p2.next = p1;
 9             p1 = p2;
10             p2 = next;
11         }
12         head.next = null;
13         return p1;
14     }
15 }

基本递归

Time: O(N), Space: O(N)递归栈大小

 1 class Solution {
 2     public ListNode reverseList(ListNode head) {
 3         if (head == null) return null;
 4         List<ListNode> res = new ArrayList<>();
 5         res.add(null);
 6         reverse(head, res);
 7         return res.get(0);
 8     }
 9     
10     public ListNode reverse(ListNode node, List<ListNode> res) {
11         if (node.next == null) {
12             res.set(0, node);
13             return node;
14         }
15         ListNode afterReverse = reverse(node.next, res);
16         afterReverse.next = node;
17         node.next = null;
18         return node;
19     }
20 }

posted @ 2015-12-15 11:15 neverlandly 阅读(268) 评论(0) 收藏举报

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