将两个已排序的链表合并

struct ListNode {
int val;
struct ListNode *next;
};

struct ListNode *mergeTwoLists(struct ListNode *l1, struct ListNode *l2) {
if (l1 == NULL)
return l2;
else if (l2 == NULL)
return l1;

struct ListNode *l3 = NULL;if(l1->val < l2->val){    l3 = l1;    l3->next = mergeTwoLists(l1->next,l2);}else{    l3 = l2;    l3->next = mergeTwoLists(l1,l2->next);}return l3;

}