/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */

struct ListNode* setDummyHead(struct ListNode* l)
{
    struct ListNode* dummyHead = malloc(sizeof(struct ListNode));
    dummyHead->val = 0;
    dummyHead->next = l;
    return dummyHead;
}

struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2){
    struct ListNode* head;
    struct ListNode* l;
    
    l1 = setDummyHead(l1);
    l2 = setDummyHead(l2);
    
    head = l1;
    l = l1;
    l1 = l1->next;
    l2 = l2->next;
    
    
    while (l1 != NULL && l2 != NULL)
    {
        if (l1->val < l2->val)
        {
            l->next = l1;
            l1 = l1->next;
        }
        else 
        {
            l->next = l2;
            l2 = l2->next;
        }
        l = l->next;
    }
    
    while (l2 != NULL)
    {
        l->next = l2;
        l2 = l2->next;
        l = l->next;
    }
    
    while (l1 != NULL)
    {
        l->next = l1;
        l1 = l1->next;
        l = l->next;
    }
    
    return head->next;
}

参考：
leetcode - 21. Merge Two Sorted Lists