Just a functional programmer playing with pointers.

main.cc

#include <vector>
#include <string>
#include <iostream>

struct Node {
    int element; 
    Node* next;  
    Node* prev;
};

Node* create (std::vector<int> elements) {
   Node* prev = new Node;
   prev->element = elements.front ();
   prev->prev = nullptr;

   Node* tip = prev;

   auto i = elements.begin ();
   elements.erase (i);
   for (auto e : elements) {
       Node* curr = new Node;
       curr->element = e;
       curr->prev = prev;
       curr->next = nullptr;
       prev->next = curr;
       prev = curr;
   }

   return tip;
}

void ppNode (Node* elem) {
    int element = elem->element;
    std::string prev = elem->prev == nullptr ? "NULL" : std::to_string(elem->prev->element);
    std::string next = elem->next == nullptr ? "NULL" : std::to_string(elem->next->element);
    std::cout << prev << " <- " << element << " -> " << next << std::endl;
}

void ppDLL (Node* list) {
    while (list != nullptr) {
        ppNode (list);
        list = list->next;
    }
}

void reverse (Node*& tip) {
    // reverse two element list 
    //   <- 1 -> <- 2 ->  <- 3 ->
    //   <- 2 -> <- 1 ->  <- 3 ->

    auto curr = tip->next;
    // last element of the list points to nothing
    tip->next = nullptr;

    while (curr != nullptr) {
        // This is the rest of the list 
        // we store for safe keeping 
        auto rest = curr->next;

        // prev should be current
        tip->prev = curr;

        // the next element should point to 
        // the head of the list
        curr->next = tip;
        curr->prev = nullptr;

        // current is now at the tip
        tip = curr;

        // start processing the rest of the list 
        // on the next iteration
        curr = rest;
    }
}

int main () {
    auto list = create ({1, 2, 3, 4, 5, 6, 7});

    reverse (list);
    ppDLL (list);
}