algorithm_dll_merge

#include <iostream>

using namespace std;

struct Node {
    int key;
    struct Node *next;
    struct Node *prev;
} *start1 = nullptr, *start2 = nullptr;


struct Node * merge_list(struct Node * head1, struct Node * head2)
{
    struct Node* head3 = nullptr;
    struct Node* p1 = head1;
    struct Node* p2 = head2;
    struct Node* p3 = nullptr;

    while (p1 != nullptr || p2 != nullptr)
    {
        struct Node * tmp = nullptr;
        if (p1 == NULL) {
            tmp = p2;
            p2 = NULL;
        } else if (p2 == nullptr) {
            tmp = p1;
            p1 = NULL;
        }

        if ((p1 != nullptr) && (p2 != nullptr)) {
            if (p1->key < p2->key) {
                tmp = p1;
                p1 = p1->next;
            } else {
                tmp = p2;
                p2 = p2->next;
            }
        }

        if (head3 == nullptr) {
            head3 = tmp;
        } else {
            p3->next = tmp;
            tmp->prev = p3;
        }
        p3 = tmp;
    }
    return head3;
}


int main() {

    return 0;
}