21. Merge Two Sorted Lists

问题https://leetcode.com/problems/merge-two-sorted-lists/description/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.思路方法一：在两条有序链表的前提下，逐个比较小的放在前面，大的放在后面，最后还有剩下的某条直接放在最后面即可；方法二：使用递归的思路解决，小的放在前面，剩下的和另一条做比较，直到递归结束，值得注意的是输入数据很长时可能会StackOverFlow；代码