21. Merge Two Sorted Lists

题目：

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.

代码如下：

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;
        }
    }
};

解体思路：

本题利用递归的思路。将两个链表归并排序，即对比两个链表返回较小值，可以分解为多个比较两个值大小的子问题。继续这个操作，即可得出答案。