Add Doubly Linked List implementation in C

C/data_structures/linked_lists/doubly_linked_list.c

@@ -0,0 +1,215 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+/*
+* insert_end : it takes the pointer to the head of the list and a data value. 
+* It creates a new node and inserts it at the end of the doubly linked list. 
+* If the list is empty, the new node becomes the head.
+* @param head: Pointer to the head pointer of the list
+* @param data: Data value to insert in the new node
+* @return: None
+*/
+
+// Structure of a node in a doubly linked list
+struct Node {
+    int data;           // Data stored in the node
+    struct Node* prev;  // Pointer to the previous node
+    struct Node* next;  // Pointer to the next node
+};
+
+// Function to create a new node
+struct Node* create_node(int data) {
+    // Allocate memory for a new node
+    struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
+    new_node->data = data;   // Set the node's data
+    new_node->prev = NULL;   // Previous is NULL for a new node
+    new_node->next = NULL;   // Next is NULL for a new node
+    return new_node;         // Return the newly created node
+}
+
+// Function to insert a node at the front (beginning) of the list
+void insert_front(struct Node** head, int data) {
+    struct Node* new_node = create_node(data); // Create a new node
+
+    new_node->next = *head;  // Make new node point to current head
+    if (*head != NULL) {
+        // If list is not empty, previous of current head becomes new node
+        (*head)->prev = new_node;
+    }
+    *head = new_node;        // Update head to point to new node
+}
+
+// Function to insert a node at the end of the list
+void insert_end(struct Node** head, int data) {
+    struct Node* new_node = create_node(data); // Create a new node
+
+    if (*head == NULL) {
+        // If list is empty, new node becomes head
+        *head = new_node;
+        return;
+    }
+
+    // Traverse to the last node
+    struct Node* temp = *head;
+    while (temp->next != NULL) {
+        temp = temp->next;
+    }
+
+    // Update links to insert new node at the end
+    temp->next = new_node;
+    new_node->prev = temp;
+}
+
+// Function to insert a node at a specific position (1-based index)
+void insert_at_position(struct Node** head, int data, int pos) {
+    if (pos <= 1 || *head == NULL) {
+        // If position is 1 or list is empty, insert at front
+        insert_front(head, data);
+        return;
+    }
+
+    struct Node* new_node = create_node(data); // Create new node
+    struct Node* temp = *head;
+
+    // Traverse to the node after which new node will be inserted
+    int i;
+    for (i = 1; i < pos - 1 && temp->next != NULL; i++) {
+        temp = temp->next;
+    }
+
+    // Update links to insert the new node
+    new_node->next = temp->next;
+    new_node->prev = temp;
+
+    if (temp->next != NULL) {
+        // If not inserting at the end, update previous pointer of next node
+        temp->next->prev = new_node;
+    }
+
+    temp->next = new_node; // Link previous node to new node
+}
+
+// Helper function to delete a node using its pointer
+void delete_node_pointer(struct Node** head, struct Node* del_node) {
+    if (*head == NULL || del_node == NULL) return; // Nothing to delete
+
+    if (*head == del_node) {
+        // If deleting the head node, move head pointer to next node
+        *head = del_node->next;
+    }
+
+    if (del_node->next != NULL) {
+        // Update previous pointer of next node
+        del_node->next->prev = del_node->prev;
+    }
+
+    if (del_node->prev != NULL) {
+        // Update next pointer of previous node
+        del_node->prev->next = del_node->next;
+    }
+
+    free(del_node); // Free memory of deleted node
+}
+
+// Function to delete a node by its value
+void delete_by_value(struct Node** head, int key) {
+    struct Node* temp = *head;
+
+    // Search for the node containing the value
+    while (temp != NULL && temp->data != key) {
+        temp = temp->next;
+    }
+
+    if (temp == NULL) {
+        // Value not found in the list
+        printf("Value %d not found in list.\n", key);
+        return;
+    }
+
+    // Delete the found node
+    delete_node_pointer(head, temp);
+}
+
+// Function to display the list from head to tail
+void display_forward(struct Node* head) {
+    struct Node* temp = head;
+    printf("Forward: ");
+    while (temp != NULL) {
+        printf("%d ", temp->data); // Print current node data
+        temp = temp->next;         // Move to next node
+    }
+    printf("\n");
+}
+
+// Function to display the list from tail to head
+void display_reverse(struct Node* head) {
+    if (head == NULL) {
+        printf("Reverse: (empty list)\n");
+        return;
+    }
+
+    struct Node* temp = head;
+
+    // Move to the last node
+    while (temp->next != NULL) {
+        temp = temp->next;
+    }
+
+    printf("Reverse: ");
+    // Traverse reverse using prev pointer
+    while (temp != NULL) {
+        printf("%d ", temp->data);
+        temp = temp->prev;
+    }
+    printf("\n");
+}
+
+// Main function to demonstrate DLL operations
+int main() {
+    struct Node* head = NULL; // Initialize empty list
+
+    // Insert nodes
+    insert_end(&head, 10);          // List: 10
+    insert_end(&head, 20);          // List: 10->20
+    insert_front(&head, 5);         // List: 5->10->20
+    insert_at_position(&head, 15, 3); // List: 5->10->15->20
+    insert_end(&head, 30);          // List: 5->10->15->20->30
+
+    // Display the list in both directions
+    display_forward(head);          // Output: 5 10 15 20 30
+    display_reverse(head);         // Output: 30 20 15 10 5
+
+    // Delete a node by value
+    delete_by_value(&head, 20);     // Delete 20
+
+    // Display after deletion
+    display_forward(head);          // Output: 5 10 15 30
+
+    // Try deleting a non-existent value
+    delete_by_value(&head, 100);    // Output: Value 100 not found in list
+
+    return 0;
+}
+/*
+ * Algorithm: Doubly Linked List (DLL)
+ * Description: A Doubly Linked List is a data structure consisting of nodes, 
+ *               where each node contains data and two pointers: 'prev' pointing 
+ *               to the previous node and 'next' pointing to the next node. This 
+ *               allows traversal in both forward and backward directions. Common 
+ *               operations include insertion (front, end, or specific position), 
+ *               deletion (by value or position), and traversal.
+ * Time Complexity:
+ *     Insertion at beginning  : O(1)
+ *     Insertion at end        : O(n)
+ *     Insertion at position   : O(n)
+ *     Deletion by value       : O(n)
+ *     Traversal (forward/reverse) : O(n)
+ * Space Complexity: O(n)  
+ * Advantages:
+ *     - Can traverse both directions
+ *     - Efficient deletion if node pointer is known
+ * Disadvantages:
+ *     - Extra memory per node for prev pointer
+ *     - More pointer manipulation can lead to bugs
+ * Author: subhahens
+ */