1public class ReverseList {
2
3 private static class ListNode {
4 int val;
5 ListNode next;
6
7 ListNode() {
8 }
9
10 ListNode(int val) {
11 this.val = val;
12 }
13
14 ListNode(int val, ListNode next) {
15 this.val = val;
16 this.next = next;
17 }
18
19
20 }
21
22 /**
23 * 206. 反转链表
24 * 反转一个单链表。
25 * <p>
26 * 示例:
27 * <p>
28 * 输入: 1->2->3->4->5->NULL
29 * 输出: 5->4->3->2->1->NULL
30 * 进阶:
31 * 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?
32 *
33 * @param head
34 * @return
35 */
36
37 //迭代
38 public static ListNode reverseList(ListNode head) {
39 ListNode temp = null;
40 ListNode pre = null, curr = head;
41 while (curr != null) {
42 temp = curr.next;
43 curr.next = pre;
44 pre = curr;
45 curr = temp;
46 }
47 return pre;
48 }
49
50 //递归
51 public static ListNode reverseList1(ListNode head) {
52 return reverse(null, head);
53 }
54
55 private static ListNode reverse(ListNode pre, ListNode curr) {
56 if (curr == null) {
57 return pre;
58 }
59 ListNode temp = null;
60 temp = curr.next;
61 curr.next = pre;
62 pre = curr;
63 curr = temp;
64 return reverse(pre, curr);
65 }
66
67 public static void main(String[] args) {
68 ListNode l4 = new ListNode(4);
69 ListNode l3 = new ListNode(3, l4);
70 ListNode l2 = new ListNode(2, l3);
71 ListNode l1 = new ListNode(1, l2);
72// reverseList(l1);
73 reverseList1(l1);
74 }
75}