两个有序单链表的合并

 1 public static Node mergeSortedList(Node head1, Node head2) {
 2         // 其中一个链表为空的情况，直接返回另一个链表头，O(1)
 3         if (head1 == null) {
 4             return head2;
 5         }
 6         if (head2 == null) {
 7             return head1;
 8         }
 9 
10         Node mergeHead = null;
11         // 先确定下来mergeHead是在哪里
12         if (head1.val < head2.val) {
13             mergeHead = head1;
14             head1 = head1.next; // 跳过已经合并了的元素
15             mergeHead.next = null; // 断开mergeHead和后面的联系
16         } else {
17             mergeHead = head2;
18             head2 = head2.next;
19             mergeHead.next = null;
20         }
21 
22         Node mergeCur = mergeHead;
23         while (head1 != null && head2 != null) {
24             if (head1.val < head2.val) {
25                 mergeCur.next = head1; // 把找到较小的元素合并到merge中
26                 head1 = head1.next; // 跳过已经合并了的元素
27                 mergeCur = mergeCur.next; // 找到下一个准备合并的元素
28                 mergeCur.next = null; // 断开mergeCur和后面的联系
29             } else {
30                 mergeCur.next = head2;
31                 head2 = head2.next;
32                 mergeCur = mergeCur.next;
33                 mergeCur.next = null;
34             }
35         }
36         // 循环结束只有一条链表还有元素
37         // 合并剩余的元素
38         if (head1 != null) {
39             mergeCur.next = head1;
40         } else if (head2 != null) {
41             mergeCur.next = head2;
42         }
43 
44         return mergeHead;
45     }

 1 public static Node mergeSortedListRec(Node head1, Node head2) {
 2         if (head1 == null) {
 3             return head2;
 4         }
 5         if (head2 == null) {
 6             return head1;
 7         }
 8 
 9         Node mergeHead = null;
10         if (head1.val < head2.val) {
11             mergeHead = head1;
12             // 连接已解决的子问题
13             mergeHead.next = mergeSortedListRec(head1.next, head2);
14         } else {
15             mergeHead = head2;
16             mergeHead.next = mergeSortedListRec(head1, head2.next);
17         }
18         return mergeHead;
19     }