21. Merge Two Sorted Lists【合并两个有序链表】

Refer: ​​​https://leetcode-cn.com/problems/merge-two-sorted-lists/solution/he-bing-liang-ge-you-xu-lian-biao-by-leetcode​​

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
          if (l1 == null) {
             return l2;
          }
          else if (l2 == null) {
             return l1;
          } else if (l1.val < l2.val) {
             l1.next = mergeTwoLists(l1.next, l2);
             return l1;
          } else {
             l2.next = mergeTwoLists(l1, l2.next);
             return l2;
          }
      }