用c语言实现给定两个链表，每个链表都已经按升序排列，设计算法实现将两个链表合并到一个升序链表中，返回合并后的链表。

#include <stdio.h>
#include <stdlib.h>

/* 链表结构体 */
typedef struct ListNode {
    int val;
    struct ListNode *next;
} ListNode;

/* 创建链表 */
ListNode* createList(int* nums, int numsSize) {
    ListNode* head = NULL;
    ListNode* tail = NULL;
    for (int i = 0; i < numsSize; i++) {
        ListNode* node = (ListNode*)malloc(sizeof(ListNode));
        node->val = nums[i];
        node->next = NULL;
        if (head == NULL) {
            head = node;
            tail = node;
        } else {
            tail->next = node;
            tail = tail->next;
        }
    }
    return head;
}

/* 合并两个链表 */
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    ListNode* head = NULL;
    ListNode* tail = NULL;
    while (l1 != NULL && l2 != NULL) {
        ListNode* node = (ListNode*)malloc(sizeof(ListNode));
        if (l1->val < l2->val) {
            node->val = l1->val;
            l1 = l1->next;
        } else {
            node->val = l2->val;
            l2 = l2->next;
        }
        node->next = NULL;
        if (head == NULL) {
            head = node;
            tail = node;
        } else {
            tail->next = node;
            tail = tail->next;
        }
    }
    if (l1 != NULL) {
        if (head == NULL) {
            head = l1;
        } else {
            tail->next = l1;
        }
    }
    if (l2 != NULL) {
        if (head == NULL) {
            head = l2;
        } else {
            tail->next = l2;
        }
    }
    return head;
}

/* 输出链表 */
void printList(ListNode* head) {
    while (head != NULL) {
        printf("%d ", head->val);
        head = head->next;
    }
    printf("\n");
}

int main() {
    int nums1[] = {1, 2, 4};
    int nums2[] = {1, 3, 4};
    ListNode* l1 = createList(nums1, 3);
    ListNode* l2 = createList(nums2, 3);
    ListNode* merged = mergeTwoLists(l1, l2);
    printList(merged);
    return 0;
}