https://leetcode.com/PRoblems/merge-two-sorted-lists/

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

連接兩個排好序（假設升序，降序類似）的鏈表，這是一道典型的遞歸題。比較兩個鏈表的第一個元素，如果l1的第一個node的值要比l2的第一個node的值小，那么新的鏈表的第一個node就是l1的第一個node，第二個node開始就是l1的第二個node和l2連接起來的list；反之同。

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {        if (l1 == null) {            return l2;        }                if (l2 == null) {            return l1;        }                if (l1.val < l2.val) {            l1.next = mergeTwoLists(l1.next, l2);            return l1;        } else {            l2.next = mergeTwoLists(l1, l2.next);            return l2;        }    }}