Min-max heap

Min-max heap
Type	binary tree/heap
Invented	1986
Time complexity in big O notation Operation Average Worst case Insert O(log n) O(log n) Delete O(log n) O(log n) Space complexity

In computer science, a min-max heap is a complete binary tree data structure which combines the usefulness of both a min-heap and a max-heap, that is, it provides constant time retrieval and logarithmic time removal of both the minimum and maximum elements in it. This makes the min-max heap a very useful data structure to implement a double-ended priority queue. Like binary min-heaps and max-heaps, min-max heaps support logarithmic insertion and deletion and can be built in linear time. Min-max heaps are often represented implicitly in an array; hence it's referred to as an implicit data structure.

The min-max heap property is: each node at an even level in the tree is less than all of its descendants, while each node at an odd level in the tree is greater than all of its descendants.

The structure can also be generalized to support other order-statistics operations efficiently, such as find-median, delete-median,find(k) (determine the kth smallest value in the structure) and the operation delete(k) (delete the kth smallest value in the structure), for any fixed value (or set of values) of k. These last two operations can be implemented in constant and logarithmic time, respectively. The notion of min-max ordering can be extended to other structures based on the max- or min-ordering, such as leftist trees, generating a new (and more powerful) class of data structures. A min-max heap can also be useful when implementing an external quicksort.

Description

• A min-max heap is a complete binary tree containing alternating min (or even) and max (or odd) levels. Even levels are for example 0, 2, 4, etc, and odd levels are respectively 1, 3, 5, etc. We assume in the next points that the root element is at the first level, i.e., 0.

Example of Min-max heap

• Each node in a min-max heap has a data member (usually called key) whose value is used to determine the order of the node in the min-max heap.
• The root element is the smallest element in the min-max heap.
• One of the two elements in the second level, which is a max (or odd) level, is the greatest element in the min-max heap
• Let $x$ be any node in a min-max heap.
  • If $x$ is on a min (or even) level, then $x.key$ is the minimum key among all keys in the subtree with root $x$.
  • If $x$ is on a max (or odd) level, then $x.key$ is the maximum key among all keys in the subtree with root $x$.
• A node on a min (max) level is called a min (max) node.

A max-min heap is defined analogously; in such a heap, the maximum value is stored at the root, and the smallest value is stored at one of the root's children.

Operations

In the following operations we assume that the min-max heap is represented in an array A[1..N]; The $ith$ location in the array will correspond to a node located on level $\lfloor \log i\rfloor$ in the heap.

Build

Creating a min-max heap is accomplished by an adaption of Floyd's linear-time heap construction algorithm, which proceeds in a bottom-up fashion. A typical Floyd's build-heap algorithm goes as follows:

function FLOYD-BUILD-HEAP (h):
    for each index i from $\lfloor length(h)/2\rfloor$ down to 1 do:
        push-down(h, i)
    return h

In this function, h is the initial array, whose elements may not be ordered according to the min-max heap property. The push-down operation (which sometimes is also called heapify) of a min-max heap is explained next.

Push Down

Hoi

Insertion

To add an element to a min-max heap perform following operations:

1. Append the required key to (the end of) the array representing the min-max heap. This will likely break the min-max heap properties, therefore we need to adjust the heap.
2. Compare the new key to its parent:
   1. If it is found to be less (greater) than the parent, then it is surely less (greater) than all other nodes on max (min) levels that are on the path to the root of heap. Now, just check for nodes on min (max) levels.
   2. The path from the new node to the root (considering only min (max) levels) should be in a descending (ascending) order and it was before the insertion. So, we need to make a binary insertion of the new node into this sequence. Technically it is simpler to swap the new node with its parent while the parent is greater (less).

Example

Here is one example for inserting an element to a Min-Max Heap.

Say we have the following min-max heap and want to insert a new node with value 6.

Initially, node 6 is inserted as a right child of the node 11. 6 is less than 11, therefore it is less than all the nodes on the max levels (41), and we need to check only the min levels (8 and 11). We should swap the nodes 6 and 11 and then swap 6 and 8. So, 6 gets moved to the root position of the heap, the former root 8 gets moved down to replace 11, and 11 becomes a right child of 8.

Consider adding the new node 81 instead of 6. Initially, the node is inserted as a right child of the node 11. 81 is greater than 11, therefore it is greater than any node on any of the min levels (8 and 11). Now, we only need to check the nodes on the max levels (41) and make one swap.

Remove Minimum

1 DeleteMin(A) 2 Input: an array A 3 Output: the previous minimum element in A 4 5 if heap-size(A) < 1 then 6 error “heap underflow” 7 min A[1] 8 A[1] A[heap-size(A)] 9 heap-size(A) heap-size(A) – 1 10 Heapify-DownMin(A, 1) 11 return min

Remove Maximum

1 DeleteMax(A) 2 Input: an array A 3 Output: the previous maximum element in A 4 5 if heap-size(A) < 1 then 6 error “heap underflow” 7 if heap-size(A) = 1 then {if there is only one element in the heap} 8 max A[1] 9 A[1] A[heap-size(A)] 10 heap-size(A) heap-size(A) – 1 11 Heapify-DownMin(A, 1) 12 else 13 if heap-size(A) < 3 or A[2] > A[3] 14 max A[2] 15 A[2] A[heap-size(A)] 16 heap-size(A) heap-size(A) – 1 17 Heapify-DownMax(A, 2) 18 else {A[3] is the maximum element} 19 max A[3] 20 A[3] A[heap-size(A)] 21 heap-size(A) heap-size(A) – 1 23 Heapify-DownMax(A, 3) 24 return max

Extensions

The min-max-median heap is a variant of the min-max heap, suggested in the original publication on the structure, that supports the operations of an order statistic tree.

References

• M. D. Atkinson, J.R. Sack, N. Santoro, and T. Strothotte, Communications of the ACM, October, 1986, Min-Max Heaps and Generalized Priority Queues.