Priorty queue

Priority Queue:
Priority Queue is similar to queue where we insert an element from the back and remove
an element from front, but with a one difference that the logical order of elements in the
priority queue depends on the priority of the elements. The element with highest priority
will be moved to the front of the queue and one with lowest priority will move to the back
of the queue. Thus it is possible that when you enqueue an element at the back in the
queue, it can move to front because of its highest priority.
Example:
Let’s say we have an array of 5 elements : {4, 8, 1, 7, 3} and we have to insert all the
elements in the max-priority queue.
First as the priority queue is empty, so 4 will be inserted initially.
Now when 8 will be inserted it will moved to front as 8 is greater than 4.
While inserting 1, as it is the current minimum element in the priority queue, it will
remain in the back of priority queue.
Now 7 will be inserted between 8 and 4 as 7 is smaller than 8.
Now 3 will be inserted before 1 as it is the 2nd minimum element in the priority queue. All
the steps are represented in the diagram below:
Naïve Approach:
Suppose we have N elements and we have to insert these elements in the priority
queue. We can use list and can insert elements in O(N) time and can sort them to
maintain a priority queue in O(N logN ) time.

Efficient Approach:
We can use heaps to implement the priority queue. It will take O(log N) time to insert
and delete each element in the priority queue.
Based on heap structure, priority queue also has two types max- priority queue and min
- priority queue. Max Priority Queue is based on the structure of max heap and can
perform following operations: maximum(Arr) : It returns maximum element from the Arr.
extract_maximum (Arr) - It removes and return the maximum element from the Arr.
increase_val (Arr, i , val) - It increases the key of element stored at index i in Arr to new
value val. insert_val (Arr, val ) - It inserts the element with value val in Arr.
Implementation:
length = number of elements in Arr.
Maximum :
int maximum(int Arr[ ])
{
return Arr[ 1 ]; //as the maximum element is the root element in
the max heap.
}
Complexity: O(1)
Extract Maximum: In this operation, the maximum element will be returned and the
last element of heap will be placed at index 1 and max_heapify will be performed on
node 1 as placing last element on index 1 will violate the property of max-heap.
int extract_maximum (int Arr[ ])
{
if(length == 0)
{
cout<< “Can’t remove element as queue is empty”;
return -1;
}
int max = Arr[1];
Arr[1] = Arr[length];
length = length -1;
max_heapify(Arr, 1);
return max;
}
Complexity: O(logN).

Increase Value: In case increasing value of any node, may violate the property of
max-heap, so we will swap the parent’s value with the node’s value until we get a larger
value on parent node.
void increase_value (int Arr[ ], int i, int val)
{
if(val < Arr[ i ])
{
cout<<”New value is less than current value, can’t be inserted”
<<endl;
return;
}
Arr[ i ] = val;
while( i > 1 and Arr[ i/2 ] < Arr[ i ])
{
swap|(Arr[ i/2 ], Arr[ i ]);
i = i/2;
}
}
Complexity : O(log N).
Insert Value :
void insert_value (int Arr[ ], int val)
{
length = length + 1;
Arr[ length ] = -1; //assuming all the numbers greater than 0 are to be
inserted in queue.
increase_val (Arr, length, val);
}
Complexity: O(log N).
Example:
Initially there are 5 elements in priority queue.
Operation: Insert Value(Arr, 6)
In the diagram below, inserting another element having value 6 is violating the property
of max-priority queue, so it is swapped with its parent having value 4, thus maintaining
the max priority queue.

Operation: Extract Maximum:
In the diagram below, after removing 8 and placing 4 at node 1, violates the property of
max-priority queue. So max_heapify(Arr, 1) will be performed which will maintain the
property of max - priority queue.
As discussed above, like heaps we can use priority queues in scheduling of jobs. When
there are N jobs in queue, each having its own priority. If the job with maximum priority
will be completed first and will be removed from the queue, we can use priority queue’s
operation extract_maximum here. If at every instant we have to add a new job in the
queue, we can use insert_value operation as it will insert the element in O(log N) and
will also maintain the property of max heap.

Priorty queue

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