src.c

#include <stdio.h>
#include <stdlib.h>
struct ListNode {
    int val;
    struct ListNode *next;

};
struct ListNode *mergeTwoLists(struct ListNode *l1, struct ListNode *l2) {
	struct ListNode *p = l1, *q = l2, *result = NULL, *k = NULL;
	if (l1 == NULL)
		return l2;
	if (l2 == NULL)
		return l1;
	if (p -> val <= q->val) {
		result = p;
		p = p->next;
	} else {
		result = q;
		q = q->next;
	}
	k = result;
	while (p && q) {
		if (p->val <= q->val) {
			k->next = p;
			p = p->next;
		} else {
			k->next = q;
			q = q->next;
		}
		k = k->next;
	}
	while (p) {
		k->next = p;
		p = p->next;
		k = k->next;
	}
	while (q) {
		k->next = q;
		q = q->next;
		k = k->next;
	}
	return result;
}
int main(int argc, char **argv)
{
	struct ListNode *l1 = NULL, *l2 = NULL;
	l1 = malloc(sizeof(*l1));
	l1->val = 2;
	l1->next = malloc(sizeof(*l1));
	l1->next->val = 4;
	l1->next->next = NULL;

	l2 = malloc(sizeof(*l2));
	l2->val = 1;
	l2->next = malloc(sizeof(*l2));
	l2->next->val = 3;
	l2->next->next = NULL;
	struct ListNode *result = mergeTwoLists(l1, l2);
	while(result) {
		printf("%d ", result->val);
		result = result->next;
	}
	return 0;
}