C Academy · Lesson

Doubly Linked Lists

Two-way links.

Lesson 4 of 413 steps

Doubly Linked Lists is a free C Academy lesson on CoddyKit. This is lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, and your progress syncs across the web and the CoddyKit app. The C Academy course includes 4 lessons in total.

Two-way links

A doubly linked list gives each node two pointers: one to the next node and one to the prev (previous) node.

This lets you walk the list in both directions and simplifies deletion.

#include <stdio.h>

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

int main(void) {
    printf("Each node links forward and backward\n");
    return 0;
}

Defining the node

The struct adds a prev pointer alongside next. Both are NULL at the ends of the list.

#include <stdio.h>
#include <stdlib.h>

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

int main(void) {
    struct Node *n = malloc(sizeof(struct Node));
    n->value = 1; n->prev = NULL; n->next = NULL;
    printf("%d\n", n->value);
    free(n);
    return 0;
}

A creation helper

As before, a helper centralizes allocation. It sets both prev and next to NULL.

#include <stdio.h>
#include <stdlib.h>

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

struct Node *make(int v) {
    struct Node *n = malloc(sizeof(struct Node));
    n->value = v; n->prev = NULL; n->next = NULL;
    return n;
}

int main(void) {
    struct Node *n = make(42);
    printf("%d\n", n->value);
    free(n);
    return 0;
}

Linking nodes both ways

When connecting two nodes you must update both directions: the first node's next and the second node's prev.

#include <stdio.h>
#include <stdlib.h>

struct Node { int value; struct Node *prev; struct Node *next; };
struct Node *make(int v){struct Node*n=malloc(sizeof*n);n->value=v;n->prev=NULL;n->next=NULL;return n;}

int main(void) {
    struct Node *a = make(1), *b = make(2);
    a->next = b;
    b->prev = a;
    printf("forward %d, back %d\n", a->next->value, b->prev->value);
    free(a); free(b);
    return 0;
}

Insert at the front

Pushing onto the front: the new node's next is the old head, the old head's prev is the new node, then the head moves to the new node.

#include <stdio.h>
#include <stdlib.h>

struct Node { int value; struct Node *prev; struct Node *next; };
struct Node *make(int v){struct Node*n=malloc(sizeof*n);n->value=v;n->prev=NULL;n->next=NULL;return n;}

void push(struct Node **head, int v) {
    struct Node *n = make(v);
    n->next = *head;
    if (*head) (*head)->prev = n;
    *head = n;
}

int main(void) {
    struct Node *head = NULL;
    push(&head, 2); push(&head, 1);
    printf("%d %d\n", head->value, head->next->value);
    return 0;
}

Forward traversal

Walking forward is identical to a singly linked list: follow next until NULL.

#include <stdio.h>
#include <stdlib.h>

struct Node { int value; struct Node *prev; struct Node *next; };
struct Node *make(int v){struct Node*n=malloc(sizeof*n);n->value=v;n->prev=NULL;n->next=NULL;return n;}

int main(void) {
    struct Node *a = make(1), *b = make(2);
    a->next = b; b->prev = a;
    for (struct Node *p = a; p; p = p->next) printf("%d ", p->value);
    printf("\n");
    free(a); free(b);
    return 0;
}

Backward traversal

The big advantage: from any node you can walk backward by following prev pointers until you reach the head.

#include <stdio.h>
#include <stdlib.h>

struct Node { int value; struct Node *prev; struct Node *next; };
struct Node *make(int v){struct Node*n=malloc(sizeof*n);n->value=v;n->prev=NULL;n->next=NULL;return n;}

int main(void) {
    struct Node *a = make(1), *b = make(2), *c = make(3);
    a->next = b; b->prev = a; b->next = c; c->prev = b;
    for (struct Node *p = c; p; p = p->prev) printf("%d ", p->value);
    printf("\n");
    free(a); free(b); free(c);
    return 0;
}

Deletion is easier

Because each node knows its predecessor, you can delete it without searching for the previous node.

Just connect node->prev to node->next in both directions.

#include <stdio.h>
#include <stdlib.h>

struct Node { int value; struct Node *prev; struct Node *next; };
struct Node *make(int v){struct Node*n=malloc(sizeof*n);n->value=v;n->prev=NULL;n->next=NULL;return n;}

void del(struct Node **head, struct Node *n) {
    if (n->prev) n->prev->next = n->next; else *head = n->next;
    if (n->next) n->next->prev = n->prev;
    free(n);
}

int main(void) {
    struct Node *a = make(1), *b = make(2), *c = make(3);
    a->next=b; b->prev=a; b->next=c; c->prev=b;
    struct Node *head = a;
    del(&head, b);
    printf("%d %d\n", head->value, head->next->value);
    return 0;
}

Update both neighbors

When removing a node, always fix the next of the previous node and the prev of the following node.

Check for NULL at each end so you do not dereference a missing neighbor.

#include <stdio.h>
#include <stdlib.h>

struct Node { int value; struct Node *prev; struct Node *next; };
struct Node *make(int v){struct Node*n=malloc(sizeof*n);n->value=v;n->prev=NULL;n->next=NULL;return n;}

int main(void) {
    struct Node *a = make(1), *b = make(2);
    a->next = b; b->prev = a;
    a->next = NULL;
    free(b);
    printf("now only %d remains\n", a->value);
    free(a);
    return 0;
}

Keeping a tail pointer

Many doubly linked lists also store a tail pointer to the last node, enabling O(1) appends and backward iteration from the end.

#include <stdio.h>
#include <stdlib.h>

struct Node { int value; struct Node *prev; struct Node *next; };
struct Node *make(int v){struct Node*n=malloc(sizeof*n);n->value=v;n->prev=NULL;n->next=NULL;return n;}

int main(void) {
    struct Node *head = make(1), *tail = head;
    struct Node *n = make(2);
    tail->next = n; n->prev = tail; tail = n;
    printf("tail = %d\n", tail->value);
    free(head); free(n);
    return 0;
}

Trade-offs

Doubly linked lists cost extra memory (one more pointer per node) and require updating two links on every change.

In return you get bidirectional traversal and O(1) deletion of a known node. Choose based on your needs.

#include <stdio.h>

int main(void) {
    printf("Singly: less memory, one-way\n");
    printf("Doubly: more memory, two-way + easy delete\n");
    return 0;
}

Quick Check

Test your understanding of doubly linked lists.

Answer it in the interactive lesson

Recap

You learned doubly linked lists:

Each node has both prev and next pointers.
Linking requires updating both directions.
You can traverse forward and backward, and delete a known node in O(1).
The cost is extra memory and more pointer updates; a tail pointer enables O(1) appends.

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