代码实现：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* merge(struct ListNode *l1, struct ListNode *l2) {
    if (l1 == NULL) {
        return l2;
    }
    if (l2 == NULL) {
        return l1;
    }
    struct ListNode *head = malloc(sizeof(*head)); // 设置虚拟头结点
    struct ListNode *tail = head;
    while (l1 && l2) {
        if (l1->val < l2->val) {
            tail->next = l1;
            l1 = l1->next;
        } else {
            tail->next = l2;
            l2 = l2->next;
        }
        tail = tail->next;
        tail->next = NULL;
    }
    if (l1) {
        tail->next = l1;
    }
    if (l2) {
        tail->next = l2;
    }
    struct ListNode *result = head->next;
    head->next = NULL;
    free(head);
    return result;   
}

struct ListNode* mergeKLists(struct ListNode **lists, int listsSize){
    if (lists == NULL || listsSize == 0) {
        return NULL;
    }
    struct ListNode *h = NULL;
    for (int i = 0; i < listsSize; i++) {
        h = merge(lists[i], h);
    }
    return h;
}