Heap

Heap

A heap is a [binary tree](../Binary Tree/) that lives inside an array, so it doesn't use parent/child pointers. The tree is partially sorted according to something called the "heap property" that determines the order of the nodes in the tree.

Common uses for heap:

• For building [priority queues](../Priority Queue/).
• The heap is the data structure supporting [heap sort](../Heap Sort/).
• Heaps are fast for when you often need to compute the minimum (or maximum) element of a collection.
• Impressing your non-programmer friends.

The heap property

There are two kinds of heaps: a max-heap and a min-heap. They are identical, except that the order in which they store the tree nodes is opposite.

In a max-heap, parent nodes must always have a greater value than each of their children. For a min-heap it's the other way around: every parent node has a smaller value than its child nodes. This is called the "heap property" and it is true for every single node in the tree.

An example:

This is a max-heap because every parent node is greater than its children. (10) is greater than (7) and (2). (7) is greater than (5) and (1).

As a result of this heap property, a max-heap always stores its largest item at the root of the tree. For a min-heap, the root is always the smallest item in the tree. That is very useful because heaps are often used as a [priority queue](../Priority Queue/) where you want to quickly access the "most important" element.

Note: You can't really say anything else about the sort order of the heap. For example, in a max-heap the maximum element is always at index 0 but the minimum element isn’t necessarily the last one -- the only guarantee you have is that it is one of the leaf nodes, but not which one.

How does heap compare to regular trees?

A heap isn't intended to be a replacement for a binary search tree. But there are many similarities between the two and also some differences. Here are some of the bigger differences:

Order of the nodes. In a [binary search tree (BST)](../Binary Search Tree/), the left child must always be smaller than its parent and the right child must be greater. This is not true for a heap. In max-heap both children must be smaller; in a min-heap they both must be greater.

Memory. Traditional trees take up more memory than just the data they store. You need to allocate additional storage for the node objects and pointers to the left/right child nodes. A heap only uses a plain array for storage and uses no pointers.

Balancing. A binary search tree must be "balanced" so that most operations have O(log n) performance. You can either insert and delete your data in a random order or use something like an [AVL tree](../AVL Tree/) or [red-black tree](../Red-Black Tree/). But with heaps we don't actually need the entire tree to be sorted. We just want the heap property to be fulfilled, and so balancing isn't an issue. Because of the way the heap is structured, heaps can guarantee O(log n) performance.

Searching. Searching a binary tree is really fast -- that's its whole purpose. In a heap, searching is slow. The purpose of a heap is to always put the largest (or smallest) node at the front, and to allow relatively fast inserts and deletes. Searching isn't a top priority.

The tree that lived in an array

An array may seem like an odd way to implement a tree-like structure but it is very efficient in both time and space.

This is how we're going to store the tree from the above example:

[ 10, 7, 2, 5, 1 ]

That's all there is to it! We don't need any more storage than just this simple array.

So how do we know which nodes are the parents and which are the children if we're not allowed to use any pointers? Good question! There is a well-defined relationship between the array index of a tree node and the array indices of its parent and children.

If i is the index of a node, then the following formulas give the array indices of its parent and child nodes:

parent(i) = floor((i - 1)/2)
left(i)   = 2i + 1
right(i)  = 2i + 2

Note that right(i) is simply left(i) + 1. The left and right nodes are always stored right next to each other.

Let's use these formulas on the example. Fill in the array index and we should get the positions of the parent and child nodes in the array:

Node	Array index (i)	Parent index	Left child	Right child
10	0	-1	1	2
7	1	0	3	4
2	2	0	5	6
5	3	1	7	8
1	4	1	9	10

Verify for yourself that these array indices indeed correspond to the picture of the tree.

Note: The root node (10) doesn't have a parent because -1 is not a valid array index. Likewise, nodes (2), (5), and (1) don't have children because those indices are greater than the array size. So we always have to make sure the indices we calculate are actually valid before we use them.

Recall that in a max-heap, the parent's value is always greater than (or equal to) the values of its children. This means the following must be true for all array indices i:

array[parent(i)] >= array[i]

Verify that this heap property holds for the array from the example heap.

As you can see, these equations let us find the parent or child index for any node without the need for pointers. True, it's slightly more complicated than just dereferencing a pointer but that's the tradeoff: we save memory space but pay with extra computations. Fortunately, the computations are fast and only take O(1) time.

It's important to understand this relationship between array index and position in the tree. Here's a slightly larger heap, this tree has 15 nodes divided over four levels:

The numbers in this picture aren't the values of the nodes but the array indices that store the nodes! Those array indices correspond to the different levels of the tree like this:

For the formulas to work, parent nodes must always appear before child nodes in the array. You can see that in the above picture.

Note that this scheme has limitations. You can do the following with a regular binary tree but not with a heap:

You can’t start a new level unless the current lowest level is completely full. So heaps always have this kind of shape:

Note: Technically speaking you could emulate a regular binary tree with a heap, but it would waste a lot of space and you’d need some way to mark array indices as being empty.

Pop quiz! Let's say we have the array:

[ 10, 14, 25, 33, 81, 82, 99 ]

Is this a valid heap? The answer is yes! A sorted array from low-to-high is a valid min-heap. We can draw this heap as follows:

The heap property holds for each node because a parent is always smaller than its children. (Verify for yourself that an array sorted from high-to-low is always a valid max-heap.)

Note: But not every min-heap is necessarily a sorted array! It only works one way... To turn a heap back into a sorted array, you need to use [heap sort](../Heap Sort/).

More math!

In case you are curious, here are a few more formulas that describe certain properties of a heap. You don't need to know these by heart but they come in handy sometimes. Feel free to skip this section!

The height of a tree is defined as the number of steps it takes to go from the root node to the lowest leaf node. Or more formally: the height is the maximum number of edges between the nodes. A heap of height h has h + 1 levels.

This heap has height 3 and therefore has 4 levels:

A heap with n nodes has height h = floor(log_2(n)). This is because we always fill up the lowest level completely before we add a new level. The example has 15 nodes, so the height is floor(log_2(15)) = floor(3.91) = 3.

If the lowest level is completely full, then that level contains 2^h nodes. The rest of the tree above it contains 2^h - 1 nodes. Fill in the numbers from the example: the lowest level has 8 nodes, which indeed is 2^3 = 8. The first three levels contain a total of 7 nodes, i.e. 2^3 - 1 = 8 - 1 = 7.

The total number of nodes n in the entire heap is therefore 2^(h+1) - 1. In the example, 2^4 - 1 = 16 - 1 = 15.

There are at most ceil(n/2^(h+1)) nodes of height h in an n-element heap.

The leaf nodes are always located at array indices floor(n/2) to n-1. We will make use of this fact to quickly build up the heap from an array. Verify this for the example if you don't believe it. ;-)

Just a few math facts to brighten your day.

What can you do with a heap?

There are two primitive operations necessary to make sure the heap is a valid max-heap or min-heap after you insert or remove an element:

• shiftUp(): If the element is greater (max-heap) or smaller (min-heap) than its parent, it needs to be swapped with the parent. This makes it move up the tree.

• shiftDown(). If the element is smaller (max-heap) or greater (min-heap) than its children, it needs to move down the tree. This operation is also called "heapify".

Shifting up or down is a recursive procedure that takes O(log n) time.

The other operations are built on these primitives. They are:

• insert(value): Adds the new element to the end of the heap and then uses shiftUp() to fix the heap.

• remove(): Removes and returns the maximum value (max-heap) or the minimum value (min-heap). To fill up the hole that's left by removing the element, the very last element is moved to the root position and then shiftDown() fixes up the heap. (This is sometimes called "extract min" or "extract max".)

• removeAtIndex(index): Just like remove() except it lets you remove any item from the heap, not just the root. This calls both shiftDown(), in case the new element is out-of-order with its children, and shiftUp(), in case the element is out-of-order with its parents.

• replace(index, value): Assigns a smaller (min-heap) or larger (max-heap) value to a node. Because this invalidates the heap property, it uses shiftUp() to patch things up. (Also called "decrease key" and "increase key".)

All of the above take time O(log n) because shifting up or down is the most expensive thing they do. There are also a few operations that take more time:

• search(value). Heaps aren't built for efficient searches but the replace() and removeAtIndex() operations require the array index of the node, so you need to find that index somehow. Time: O(n).

• buildHeap(array): Converts an (unsorted) array into a heap by repeatedly calling insert(). If you’re smart about this, it can be done in O(n) time.

• [Heap sort](../Heap Sort/). Since the heap is really an array, we can use its unique properties to sort the array from low to high. Time: O(n lg n).

The heap also has a peek() function that returns the maximum (max-heap) or minimum (min-heap) element, without removing it from the heap. Time: O(1).

Note: By far the most common things you'll do with a heap are inserting new values with insert() and removing the maximum or minimum value with remove(). Both take O(log n) time. The other operations exist to support more advanced usage, such as building a priority queue where the "importance" of items can change after they've been added to the queue.

Inserting into the heap

Let's go through an example insertion to see in more detail how this works. We'll insert the value 16 into this heap:

The array for this heap is [ 10, 7, 2, 5, 1 ].

The first step when inserting a new item is to append it to the end of the array. The array becomes:

[ 10, 7, 2, 5, 1, 16 ]

This corresponds to the following tree:

The (16) was added to the first available space on the last row.

Unfortunately, the heap property is no longer satisfied because (2) is above (16) and we want higher numbers above lower numbers. (This is a max-heap.)

To restore the heap property, we're going to swap (16) and (2).

We're not done yet because (10) is also smaller than (16). We keep swapping our inserted value with its parent, until the parent is larger or we reach the top of the tree. This is called shift-up or sifting and is done after every insertion. It makes a number that is too large or too small "float up" the tree.

Finally, we get:

And now every parent is greater than its children again.

The time required for shifting up is proportional to the height of the tree so it takes O(log n) time. (The time it takes to append the node to the end of the array is only O(1), so that doesn't slow it down.)

Removing the root

Let's remove (10) from this tree:

What happens to the empty spot at the top?

When inserting, we put the new value at the end of the array. Here, we'll do the opposite: we're going to take the last object we have, stick it up on top of the tree, and restore the heap property.

Let's look at how to shift-down (1). To maintain the heap property for this max-heap, we want to the highest number of top. We have two candidates for swapping places with: (7) and (2). We choose the highest number between these three nodes to be on top. That is (7), so swapping (1) and (7) gives us the following tree.

Keep shifting down until the node doesn't have any children or it is larger than both its children. For our heap we only need one more swap to restore the heap property:

The time required for shifting all the way down is proportional to the height of the tree so it takes O(log n) time.

Note: shiftUp() and shiftDown() can only fix one out-of-place element at a time. If there are multiple elements in the wrong place, you need to call these functions once for each of those elements.

Removing any node

The vast majority of the time you'll be removing the object at the root of the heap because that's what heaps are designed for.

However, it can be useful to remove an arbitrary element. This is a more general version of remove() and may involve either shiftDown() or shiftUp().

Let's take the example tree again and remove (7):

As a reminder, the array is:

[ 10, 7, 2, 5, 1 ]

As you know, removing an element could potentially invalidate the max-heap or min-heap property. To fix this, we swap the node that we're removing with the last element:

[ 10, 1, 2, 5, 7 ]

The last element is the one that we'll return; we'll call removeLast() to remove it from the heap. The (1) is now out-of-order because it's smaller than its child, (5), but sits higher in the tree. We call shiftDown() to repair this.

However, shifting down is not the only situation we need to handle -- it may also happen that the new element must be shifted up. Consider what happens if you remove (5) from the following heap:

Now (5) gets swapped with (8). Because (8) is larger than its parent, we need to call shiftUp().

Creating a heap from an array

It can be convenient to convert an array into a heap. This just shuffles the array elements around until the heap property is satisfied.

In code it would look like this:

  private mutating func buildHeap(fromArray array: [T]) {
    for value in array {
      insert(value)
    }
  }

We simply call insert() for each of the values in the array. Simple enough, but not very efficient. This takes O(n log n) time in total because there are n elements and each insertion takes log n time.

If you didn't gloss over the math section, you'd have seen that for any heap the elements at array indices n/2 to n-1 are the leaves of the tree. We can simply skip those leaves. We only have to process the other nodes, since they are parents with one or more children and therefore may be in the wrong order.

In code:

  private mutating func buildHeap(fromArray array: [T]) {
    elements = array
    for i in (elements.count/2 - 1).stride(through: 0, by: -1) {
      shiftDown(index: i, heapSize: elements.count)
    }
  }

Here, elements is the heap's own array. We walk backwards through this array, starting at the first non-leaf node, and call shiftDown(). This simple loop puts these nodes, as well as the leaves that we skipped, in the correct order. This is known as Floyd's algorithm and only takes O(n) time. Win!

Searching the heap

Heaps aren't made for fast searches, but if you want to remove an arbitrary element using removeAtIndex() or change the value of an element with replace(), then you need to obtain the index of that element somehow. Searching is one way to do this but it's kind of slow.

In a [binary search tree](../Binary Search Tree/) you can depend on the order of the nodes to guarantee a fast search. A heap orders its nodes differently and so a binary search won't work. You'll potentially have to look at every node in the tree.

Let's take our example heap again:

If we want to search for the index of node (1), we could just step through the array [ 10, 7, 2, 5, 1 ] with a linear search.

But even though the heap property wasn't conceived with searching in mind, we can still take advantage of it. We know that in a max-heap a parent node is always larger than its children, so we can ignore those children (and their children, and so on...) if the parent is already smaller than the value we're looking for.

Let's say we want to see if the heap contains the value 8 (it doesn't). We start at the root (10). This is obviously not what we're looking for, so we recursively look at its left and right child. The left child is (7). That is also not what we want, but since this is a max-heap, we know there's no point in looking at the children of (7). They will always be smaller than 7 and are therefore never equal to 8. Likewise for the right child, (2).

Despite this small optimization, searching is still an O(n) operation.

Note: There is away to turn lookups into a O(1) operation by keeping an additional dictionary that maps node values to indices. This may be worth doing if you often need to call replace() to change the "priority" of objects in a [priority queue](../Priority Queue/) that's built on a heap.

The code

See Heap.swift for the implementation of these concepts in Swift. Most of the code is quite straightforward. The only tricky bits are in shiftUp() and shiftDown().

You've seen that there are two types of heaps: a max-heap and a min-heap. The only difference between them is in how they order their nodes: largest value first or smallest value first.

Rather than create two different versions, MaxHeap and MinHeap, there is just one Heap object and it takes an isOrderedBefore closure. This closure contains the logic that determines the order of two values. You've probably seen this before because it's also how Swift's sort() works.

To make a max-heap of integers, you write:

var maxHeap = Heap<Int>(sort: >)

And to create a min-heap you write:

var minHeap = Heap<Int>(sort: <)

I just wanted to point this out, because where most heap implementations use the < and > operators to compare values, this one uses the isOrderedBefore() closure.

See also

Heap on Wikipedia

Written for the Swift Algorithm Club by Kevin Randrup and Matthijs Hollemans