Priority Queue

• Insert - adds a new element
• deleteMin - delete minimum element in priority queue

Applications of Priority Queues

• Operating systems : shortest job first process scheduling
• Simulators : Scheduling the next event (Smallest event time)

Implementation

• Linked Lists - O(N) worst case on insert or deleteMin()
• Binary Search Trees - O(LogN) average time complexity on insert and delete
• Overkill 0 all elements are sorted
• Heaps - O(logN) worst case for both insertion and delete operations
  • Does not guarantee that the order is still first in first out

Partially ordered Trees

• Partially ordered tree (POT) is a tree such that:
• There is an order relation <= defined for the vertices of T
• For any vertex p and any child c of p, p <= c
• However, the smallest element in POT is the root. No conclusion can be drawn about the order of the children.

Binary Heaps

• A binary heap is a partially ordered complete binary tree
• The tree is completely filled on all levels except possibly the lowest
• In a more general d-Heap
  • A parent node can have d children
• We simply refer to binary heaps as heaps

Vector Representation

• Parent of v[k] = v[k/2]
• Left Child of v[k] = v[2 * k]
• Right child of v[k] = v[2 * k + 1]

Basic Heap Operations : Insert (cx)

• Create a hole at the next leaf
• Repair upwards
• Repeat
  • Locate Parent
  • if POT not satisfied
  • else: stop
• Insert x into the hole

void insert(const Comparable& x)
{
	if(currSize == array.size()-1){
		array.resize(array.size() * 2)
	}
	int hole = ++currSize;
	Comparable copy = x;
	array[0] = move(copy);
	for(; x < array[hole/2]; hole /= 2){
		array[hole]	= move(array[hole / 2]);
	}
	array[hole] = move(array[0]);
}

Delete Min

void deleteMin(){
	if(isEmpty())
		throw UnderflowException{};
	array[1] = move( array [currSize--]);
	percolateDown(1);
}

percolateDown(int hole){
	int child;
	T tmp = move(array [hole] );
	for(; hole * 2 <= currSize; hole = child){
		child = hole * 2;
		if(child != currSize && array[child+1] < array[child]){
			++child;
		}
		if(array[child] < tmp)
			array[hole] = move(array[child]);
		else
			break;
	}
	array[hole] = move(tmp);
}