Doubly Circular Linked List | Set 2 (Deletion)

1.5

We have discussed doubly circular linked list introduction and its insertion.

Let us formulate the problem statement to understand the deletion process. Given a ‘key’, delete the first occurrence of this key in circular doubly linked list.

Algorithm

Case 1: Empty List(start = NULL)

  • If the list is empty, simply return.

Case 2:List initially contain some nodes, start points to first node of the List

  1. If the list is not empty then we define two pointers curr and prev_1 and initialize the pointer curr points to first node of the list and prev_1 = NULL.
  2. Traverse the list using curr pointer to find the node to be deleted and before moving curr to next node, every time set prev_1 = curr.
  3. If the node is found, check if it is the only node in the list. If yes, set start = NULL and free the node pointing by curr.
  4. If the list has more than one node, check if it is the first node of the list. Condition to check this is (curr == start). If yes, then move prev_1 to the last node(prev_1 = start -> prev). After prev_1 reaches the last node, set start = start -> next and prev_1 -> next = start and start -> prev = prev_1. Free the node pointing by curr.
  5. If curr is not first node, we check if it is the last node in the list. Condition to check this is (curr -> next == start). If yes, set prev_1 -> next = start and start -> prev = prev_1. Free the node pointing by curr.
  6. If the node to be deleted is neither the first node nor the last node, declare one more pointer temp and initialize the pointer temp points to next of curr pointer (temp = curr->next). Now set, prev_1 -> next = temp and temp ->prev = prev_1. Free the node pointing by curr.
  • If the given key(Say 4) matches with first node of the list(Step 4):
    Delete_first_node
  • If the given key(Say 8) matches with last node of the list(Step 5):
    Delete_last_node
  • If the given key(Say 6) matches with middle node of the list(Step 6):
    Delete_middle_node
// C++ program to delete a given key from
// circular doubly linked list.
#include<bits/stdc++.h>
using namespace std;

// Structure of a Node
struct Node
{
    int data;
    struct Node *next;
    struct Node *prev;
};

// Function to insert node in the list
void insert(struct Node** start, int value)
{
    // If the list is empty, create a single node
    // circular and doubly list
    if (*start == NULL)
    {
        struct Node* new_node = new Node;
        new_node->data = value;
        new_node->next = new_node->prev = new_node;
        *start = new_node;
        return;
    }

    // If list is not empty

    /* Find last node */
    Node *last = (*start)->prev;

    // Create Node dynamically
    struct Node* new_node = new Node;
    new_node->data = value;

    // Start is going to be next of new_node
    new_node->next = *start;

    // Make new node previous of start
    (*start)->prev = new_node;

    // Make last preivous of new node
    new_node->prev = last;

    // Make new node next of old last
    last->next = new_node;
}

// Function to delete a given node from the list
void deleteNode(struct Node **start, int key)
{
    // If list is empty
    if (*start == NULL)
        return;

    // Find the required node
    // Declare two pointers and initialize them
    struct Node *curr = *start, *prev_1 = NULL;
    while (curr -> data != key)
    {
        // If node is not present in the list
        if (curr->next == *start)
        {
            printf("\nList doesn't have node with value = %d", key);
            return;
        }

        prev_1 = curr;
        curr = curr -> next;
    }

    // Check if node is the only node in list
    if (curr -> next == *start && prev_1 == NULL)
    {
        (*start) = NULL;
        free(curr);
        return;
    }

    // If list has more than one node,
    // check if it is the first node
    if (curr == *start)
    {
        // Move prev_1 to last node
        prev_1 = (*start) -> prev;

        // Move start ahead
        *start = (*start) -> next;

        // Adjust the pointers of prev_1 and start node
        prev_1 -> next = *start;
        (*start) -> prev = prev_1;
        free(curr);
    }

    // check if it is the last node
    else if (curr->next == *start)
    {
        // Adjust the pointers of prev_1 and start node
        prev_1 -> next = *start;
        (*start) -> prev = prev_1;
        free(curr);
    }
    else
    {
        // create new pointer, points to next of curr node
        struct Node *temp = curr -> next;

        // Adjust the pointers of prev_1 and temp node
        prev_1 -> next = temp;
        temp -> prev = prev_1;
        free(curr);
    }
}

// Fuction to display list elements
void display(struct Node* start)
{
    struct Node *temp = start;

    while (temp->next != start)
    {
        printf("%d ", temp->data);
        temp = temp->next;
    }
    printf("%d ", temp->data);
}

// Driver program to test above functions
int main()
{
    // Start with the empty list
    struct Node *start = NULL;

    // Created linked list will be 4->5->6->7->8
    insert(&start, 4);
    insert(&start, 5);
    insert(&start, 6);
    insert(&start, 7);
    insert(&start, 8);

    printf("List Before Deletion: ");
    display(start);

    // Delete the node which is not present in list
    deleteNode(&start, 9);
    printf("\nList After Deletion: ");
    display(start);

    // Delete the first node
    deleteNode(&start, 4);
    printf("\nList After Deleting %d: ", 4);
    display(start);

    // Delete the last node
    deleteNode(&start, 8);
    printf("\nList After Deleting %d: ", 8);
    display(start);

    // Delete the middle node
    deleteNode(&start, 6);
    printf("\nList After Deleting %d: ", 6);
    display(start);

    return 0;
}

Output:

List Before Deletion: 4 5 6 7 8 
List doesn't have node with value = 9
List After Deletion: 4 5 6 7 8 
List After Deleting 4: 5 6 7 8 
List After Deleting 8: 5 6 7 
List After Deleting 6: 5 7 

This article is contributed by Akash Gupta. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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