21. 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.

注意边界，注意边界，注意边界

递归实现

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode *head = new ListNode(0);
        if (l1 == NULL) {
           if (l2 == NULL) return NULL;
           return l2;
        }
        if (l2 == NULL) {
            return l1;
        }
        if (l1->val < l2->val) {
            head->val = l1->val;
            head->next = mergeTwoLists(l1->next, l2);
        }
        else {
            head->val = l2->val;
            head->next = mergeTwoLists(l1, l2->next);
        }
        return head;
    }
};