Fibonacci Heap - Deletion, Extract min and Decrease key

Fibonacci Heap – Deletion, Extract min and Decrease key

Difficulty Level : Hard
Last Updated : 14 Dec, 2020

In the last post, we discussed the Insertion and Union of Fibonacci Heaps. In this post, we will discuss Extract_min(), Decrease_key() and Deletion() operations on Fibonacci heap.

Prerequisites:
Fibonacci Heap (Introduction)
Fibonacci Heap – Insertion and Union

Extract_min(): We create a function for deleting the minimum node and setting the min pointer to the minimum value in the remaining heap. The following algorithm is followed:

Delete the min node.
Set head to the next min node and add all the trees of the deleted node in the root list.
Create an array of degree pointers of the size of the deleted node.
Set degree pointer to the current node.
Move to the next node.
- If degrees are different then set degree pointer to next node.
- If degrees are the same then join the Fibonacci trees by union operation.
Repeat steps 4 and 5 until the heap is completed.

Example:

Decrease_key(): To decrease the value of any element in the heap, we follow the following algorithm:

Decrease the value of the node ‘x’ to the new chosen value.
CASE 1) If min-heap property is not violated,
- Update min pointer if necessary.
CASE 2) If min-heap property is violated and parent of ‘x’ is unmarked,
- Cut off the link between ‘x’ and its parent.
- Mark the parent of ‘x’.
- Add tree rooted at ‘x’ to the root list and update min pointer if necessary.
CASE 3)If min-heap property is violated and parent of ‘x’ is marked,
- Cut off the link between ‘x’ and its parent p[x].
- Add ‘x’ to the root list, updating min pointer if necessary.
- Cut off link between p[x] and p[p[x]].
- Add p[x] to the root list, updating min pointer if necessary.
- If p[p[x]] is unmarked, mark it.
- Else, cut off p[p[x]] and repeat steps 4.2 to 4.5, taking p[p[x]] as ‘x’.

Example:

Deletion(): To delete any element in a Fibonacci heap, the following algorithm is followed:

Decrease the value of the node to be deleted ‘x’ to a minimum by Decrease_key() function.
By using min-heap property, heapify the heap containing ‘x’, bringing ‘x’ to the root list.
Apply Extract_min() algorithm to the Fibonacci heap.

Example:

Following is a program to demonstrate Extract min(), Deletion() and Decrease key() operations on a Fibonacci Heap:

C++

// C++ program to demonstrate Extract min, Deletion()
// and Decrease key() operations in a fibonacci heap
#include <cmath>
#include <cstdlib>
#include <iostream>
#include <malloc.h>
using namespace std;
 
// Creating a structure to represent a node in the heap
struct node {
    node* parent; // Parent pointer
    node* child; // Child pointer
    node* left; // Pointer to the node on the left
    node* right; // Pointer to the node on the right
    int key; // Value of the node
    int degree; // Degree of the node
    char mark; // Black or white mark of the node
    char c; // Flag for assisting in the Find node function
};
 
// Creating min pointer as "mini"
struct node* mini = NULL;
 
// Declare an integer for number of nodes in the heap
int no_of_nodes = 0;
 
// Function to insert a node in heap
void insertion(int val)
{
    struct node* new_node = (struct node*)malloc(sizeof(struct node));
    new_node->key = val;
    new_node->degree = 0;
    new_node->mark = 'W';
    new_node->c = 'N';
    new_node->parent = NULL;
    new_node->child = NULL;
    new_node->left = new_node;
    new_node->right = new_node;
    if (mini != NULL) {
        (mini->left)->right = new_node;
        new_node->right = mini;
        new_node->left = mini->left;
        mini->left = new_node;
        if (new_node->key < mini->key)
            mini = new_node;
    }
    else {
        mini = new_node;
    }
    no_of_nodes++;
}
// Linking the heap nodes in parent child relationship
void Fibonnaci_link(struct node* ptr2, struct node* ptr1)
{
    (ptr2->left)->right = ptr2->right;
    (ptr2->right)->left = ptr2->left;
    if (ptr1->right == ptr1)
        mini = ptr1;
    ptr2->left = ptr2;
    ptr2->right = ptr2;
    ptr2->parent = ptr1;
    if (ptr1->child == NULL)
        ptr1->child = ptr2;
    ptr2->right = ptr1->child;
    ptr2->left = (ptr1->child)->left;
    ((ptr1->child)->left)->right = ptr2;
    (ptr1->child)->left = ptr2;
    if (ptr2->key < (ptr1->child)->key)
        ptr1->child = ptr2;
    ptr1->degree++;
}
// Consolidating the heap
void Consolidate()
{
    int temp1;
    float temp2 = (log(no_of_nodes)) / (log(2));
    int temp3 = temp2;
    struct node* arr[temp3];
    for (int i = 0; i <= temp3; i++)
        arr[i] = NULL;
    node* ptr1 = mini;
    node* ptr2;
    node* ptr3;
    node* ptr4 = ptr1;
    do {
        ptr4 = ptr4->right;
        temp1 = ptr1->degree;
        while (arr[temp1] != NULL) {
            ptr2 = arr[temp1];
            if (ptr1->key > ptr2->key) {
                ptr3 = ptr1;
                ptr1 = ptr2;
                ptr2 = ptr3;
            }
            if (ptr2 == mini)
                mini = ptr1;
            Fibonnaci_link(ptr2, ptr1);
            if (ptr1->right == ptr1)
                mini = ptr1;
            arr[temp1] = NULL;
            temp1++;
        }
        arr[temp1] = ptr1;
        ptr1 = ptr1->right;
    } while (ptr1 != mini);
    mini = NULL;
    for (int j = 0; j <= temp3; j++) {
        if (arr[j] != NULL) {
            arr[j]->left = arr[j];
            arr[j]->right = arr[j];
            if (mini != NULL) {
                (mini->left)->right = arr[j];
                arr[j]->right = mini;
                arr[j]->left = mini->left;
                mini->left = arr[j];
                if (arr[j]->key < mini->key)
                    mini = arr[j];
            }
            else {
                mini = arr[j];
            }
            if (mini == NULL)
                mini = arr[j];
            else if (arr[j]->key < mini->key)
                mini = arr[j];
        }
    }
}
 
// Function to extract minimum node in the heap
void Extract_min()
{
    if (mini == NULL)
        cout << "The heap is empty" << endl;
    else {
        node* temp = mini;
        node* pntr;
        pntr = temp;
        node* x = NULL;
        if (temp->child != NULL) {
 
            x = temp->child;
            do {
                pntr = x->right;
                (mini->left)->right = x;
                x->right = mini;
                x->left = mini->left;
                mini->left = x;
                if (x->key < mini->key)
                    mini = x;
                x->parent = NULL;
                x = pntr;
            } while (pntr != temp->child);
        }
        (temp->left)->right = temp->right;
        (temp->right)->left = temp->left;
        mini = temp->right;
        if (temp == temp->right && temp->child == NULL)
            mini = NULL;
        else {
            mini = temp->right;
            Consolidate();
        }
        no_of_nodes--;
    }
}
 
// Cutting a node in the heap to be placed in the root list
void Cut(struct node* found, struct node* temp)
{
    if (found == found->right)
        temp->child = NULL;
 
    (found->left)->right = found->right;
    (found->right)->left = found->left;
    if (found == temp->child)
        temp->child = found->right;
 
    temp->degree = temp->degree - 1;
    found->right = found;
    found->left = found;
    (mini->left)->right = found;
    found->right = mini;
    found->left = mini->left;
    mini->left = found;
    found->parent = NULL;
    found->mark = 'B';
}
 
// Recursive cascade cutting function
void Cascase_cut(struct node* temp)
{
    node* ptr5 = temp->parent;
    if (ptr5 != NULL) {
        if (temp->mark == 'W') {
            temp->mark = 'B';
        }
        else {
            Cut(temp, ptr5);
            Cascase_cut(ptr5);
        }
    }
}
 
// Function to decrease the value of a node in the heap
void Decrease_key(struct node* found, int val)
{
    if (mini == NULL)
        cout << "The Heap is Empty" << endl;
 
    if (found == NULL)
        cout << "Node not found in the Heap" << endl;
 
    found->key = val;
 
    struct node* temp = found->parent;
    if (temp != NULL && found->key < temp->key) {
        Cut(found, temp);
        Cascase_cut(temp);
    }
    if (found->key < mini->key)
        mini = found;
}
 
// Function to find the given node
void Find(struct node* mini, int old_val, int val)
{
    struct node* found = NULL;
    node* temp5 = mini;
    temp5->c = 'Y';
    node* found_ptr = NULL;
    if (temp5->key == old_val) {
        found_ptr = temp5;
        temp5->c = 'N';
        found = found_ptr;
        Decrease_key(found, val);
    }
    if (found_ptr == NULL) {
        if (temp5->child != NULL)
            Find(temp5->child, old_val, val);
        if ((temp5->right)->c != 'Y')
            Find(temp5->right, old_val, val);
    }
    temp5->c = 'N';
    found = found_ptr;
}
 
// Deleting a node from the heap
void Deletion(int val)
{
    if (mini == NULL)
        cout << "The heap is empty" << endl;
    else {
 
        // Decreasing the value of the node to 0
        Find(mini, val, 0);
 
        // Calling Extract_min function to
        // delete minimum value node, which is 0
        Extract_min();
        cout << "Key Deleted" << endl;
    }
}
 
// Function to display the heap
void display()
{
    node* ptr = mini;
    if (ptr == NULL)
        cout << "The Heap is Empty" << endl;
 
    else {
        cout << "The root nodes of Heap are: " << endl;
        do {
            cout << ptr->key;
            ptr = ptr->right;
            if (ptr != mini) {
                cout << "-->";
            }
        } while (ptr != mini && ptr->right != NULL);
        cout << endl
             << "The heap has " << no_of_nodes << " nodes" << endl
             << endl;
    }
}
 
// Driver code
int main()
{
    // We will create a heap and insert 3 nodes into it
    cout << "Creating an initial heap" << endl;
    insertion(5);
    insertion(2);
    insertion(8);
 
    // Now we will display the root list of the heap
    display();
 
    // Now we will extract the minimum value node from the heap
    cout << "Extracting min" << endl;
    Extract_min();
    display();
 
    // Now we will decrease the value of node '8' to '7'
    cout << "Decrease value of 8 to 7" << endl;
    Find(mini, 8, 7);
    display();
 
    // Now we will delete the node '7'
    cout << "Delete the node 7" << endl;
    Deletion(7);
    display();
 
    return 0;
}

Output:

Creating an initial heap
The root nodes of Heap are: 
2-->5-->8
The heap has 3 nodes

Extracting min
The root nodes of Heap are: 
5
The heap has 2 nodes

Decrease value of 8 to 7
The root nodes of Heap are: 
5
The heap has 2 nodes

Delete the node 7
Key Deleted
The root nodes of Heap are: 
5
The heap has 1 nodes

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