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，用c语言写出这段代码

以下是C语言代码实现：

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

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

struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2){
    struct ListNode *head = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode *cur = head;
    while(l1 && l2){
        if(l1->val <= l2->val){
            cur->next = l1;
            l1 = l1->next;
        }else{
            cur->next = l2;
            l2 = l2->next;
        }
        cur = cur->next;
    }
    cur->next = l1 ? l1 : l2;
    return head->next;
}

int main(){
    struct ListNode *l1 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode *l2 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode *cur1 = l1;
    struct ListNode *cur2 = l2;
    int arr1[3] = {1, 2, 4};
    int arr2[3] = {1, 3, 4};
    for(int i = 0; i < 3; i++){
        cur1->val = arr1[i];
        cur2->val = arr2[i];
        if(i != 2){
            cur1->next = (struct ListNode*)malloc(sizeof(struct ListNode));
            cur2->next = (struct ListNode*)malloc(sizeof(struct ListNode));
            cur1 = cur1->next;
            cur2 = cur2->next;
        }
    }
    cur1->next = NULL;
    cur2->next = NULL;
    struct ListNode *res = mergeTwoLists(l1, l2);
    while(res){
        printf("%d ", res->val);
        res = res->next;
    }
    return 0;
}

这段代码实现了将两个升序链表合并成一个升序链表的功能。