21 Merge Two Sorted Lists（easy）

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.

合并两个排好序的List 采用两个指针记录两个List的下表，比较两个指针对映的元素的大小，小的元素加入最终的List并将指针往前移动，最后注意处理剩余的元素。