Heap – Theory

Binary Heap is the underlying implementation of Priority Queue. A heap can be visualized to have a tree structure logically. This tree  satisfies the two heap properties – value property and the shape property. Although the heap is a tree, because of the heap property it’s easy to implement it as an array.
Binary Heap is an optimal DS to implement Priority Queue.

Heap Property
Heap Property
Heap Implementation using Array
Heap Implementation using Array

Time complexity of building a heap if the entire array is known beforehand = O(n) (and not O(n(logn))) – SO But if we insert one by one, then it will be O(n(log(n))).
For a binary heap we have O(log(n)) for insert, O(log(n)) for delete min.

Searching in a heap –  You need to search through every element in the heap in order to determine if an element is inside. One optimization is possible, though (we assume a max heap here). If you have reached a node with a lower value than the element you are searching for, you don’t need to search further from that node. However, even with this optimization, search is still O(N) (need to check N/2 nodes in average). SO

Heap is better at findMin/findMax (O(1)), while BST is good at all finds (O(logN)). Insert is O(logN) for both structures. If you only care about findMin/findMax (e.g. priority-related), go with heap. If you want everything sorted, go with BST.  SO

Why is Binary Heap Preferred over BST for Priority Queue? GFG

So why is Binary Heap Preferred for Priority Queue?

  • Since Binary Heap is implemented using arrays, there is always better locality of reference and operations are more cache friendly.
  • Although operations are of same time complexity, constants in Binary Search Tree are higher.
  • We can build a Binary Heap in O(n) time. Self Balancing BSTs require O(nLogn) time to construct.
  • Binary Heap doesn’t require extra space for pointers.
  • Binary Heap is easier to implement.
  • There are variations of Binary Heap like Fibonacci Heap that can support insert and decrease-key in Θ(1) time

Is Binary Heap always better?
Although Binary Heap is for Priority Queue, BSTs have their own advantages and the list of advantages is in-fact bigger compared to binary heap.

  • Searching an element in self-balancing BST is O(Logn) which is O(n) in Binary Heap.
  • We can print all elements of BST in sorted order in O(n) time, but Binary Heap requires O(nLogn) time.
  • Floor and ceil can be found in O(Logn) time.
  • K’th largest/smallest element be found in O(Logn) time by augmenting tree with an additional field.

Heapsort algorithm has limited uses because Quicksort is better in practice.

Average binary heap insert is O(1)

Intuitive argument:

  • bottom tree levels have exponentially more elements than top levels, so new elements are almost certain to go at the bottom
  • heap insertion starts from the bottom, BST must start from the top

When is heap sort used –  Although QuickSort works better in practice, the advantage of HeapSort worst case upper bound of O(nLogn). MergeSort also has upper bound as O(nLogn) and works better in practice when compared to HeapSort. But MergeSort requires O(n) extra space HeapSort is not used much in practice, but can be useful in real time (or time bound where QuickSort doesn’t fit) embedded systems where less space is available (MergeSort doesn’t fit)





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