如何在C++中合并已排序的列表

我知道这个问题已经被问了很多次，部分答案是关于合并排序的问题，但我似乎不太明白。

这是我的代码：

#include <iostream>
using namespace std;
struct listNode {
    int info;
    struct listNode *next;
};

struct listNode * addHead(struct listNode* head, int k);
void printAll(struct listNode *head);
struct listNode * deleteLast(struct listNode *head);
struct listNode * append(struct listNode *a, struct listNode *b);
struct listNode * zip(struct listNode *a, struct listNode *b);
struct listNode * merge(struct listNode *a, struct listNode*b);
int main() 
{
    listNode *list1 = 0;
    listNode *list2 = 0;
    listNode *list3 = 0;
    listNode *list4 = 0;
    listNode *list5 = 0;
    //fill list1
    list1 = addHead(list1, 6);
    list1 = addHead(list1, 4);
    list1 = addHead(list1, 2);
    //fill list2
    list2 = addHead(list2, 3);
    list2 = addHead(list2, 1);

    //test deleteLast
    cout << "List 1 contains: " << endl;
    printAll(list1);
    cout << "Deleting last node of List 1. Now contains: " << endl;
    list1 = deleteLast(list1);
    printAll(list1);
    cout << "List 2 contains: " << endl;
    printAll(list2);
    //test append
    cout << "Appending list 1 and list 2 yields: " << endl;
    list3 = append(list1, list2);
    printAll(list3);
    //zip test
    cout << "The zipped list of list 1 and list 2 is: " << endl;
    list4 = zip(list1, list2);
    printAll(list4);
    //merge test
    cout << "The merged list of list1 and list 2 is: " << endl;
    list5 = merge(list1, list2);
    printAll(list5);
    return 0;
}
struct listNode *deleteLast(struct listNode *head) {
    if (head == 0) {
        return NULL;
    } else if (head->next == 0) {
        delete head;
        return NULL;
    } else {
        head->next = deleteLast(head->next);
    }
    return head;
}
struct listNode * addHead(struct listNode *head, int k) {
    listNode *nnode = new listNode;
    nnode->info = k;
    nnode->next = head;
    return nnode;
}
void printAll(listNode *head) {
    if (head == 0) {
        cout << endl;
    } else {
        cout << head->info << "->";
        printAll(head->next);
    }
}
struct listNode * append(struct listNode *ahead,struct listNode *bhead) {
    if (bhead == 0) {
        return ahead;
    } else if (ahead == 0) {
        return bhead;
    } else {
        ahead->next = append(ahead->next, bhead);
    }
    return ahead;
}
struct listNode * zip(struct listNode *a, struct listNode *b) 
{
    if (b == 0) {
        return 0;
    } else {
        listNode *tmp = a->next;
        a->next = b;
        a->next = zip(b, tmp);
    }
    return a;
}
struct listNode *merge(struct listNode *a, struct listNode *b)
{
    listNode *result = NULL;
    if (a == NULL)
        return b;
    else if (b == NULL)
        return a;
    if (a->info < b->info) {
        result = a;
        result->next = merge(a->next, b);
    } else {
        result = b;
        result -> next = merge(a, b->next);
    }
    return result;
}

现在它就是不能正常工作。我陷入了一个无限循环，然后出现了分段错误。有人能告诉我我做错了什么吗？