021Merge Two Sorted Lists

合并两个有序链表，链表问题建立一个伪头结点很好使，当前指针从p从p.next开始，返回head.next

代码如下：

ListNode head = new ListNode(0);ListNode p = head;while (l1 != null || l2 != null) {if (l1 == null) {p.next = l2;break;}if (l2 == null) {p.next = l1;break;}if (l1 != null && l2 != null) {if (l1.val > l2.val) {p.next = l2;p = p.next;l2 = l2.next;} else {p.next = l1;p = p.next;l1 = l1.next;}}}return head.next;