Question

Write an algorithm that determines whether or not an almost complete binary tree is a heap.

Short answer

The algorithm to check if an almost complete binary tree is a heap would involve two main checks: That the tree meets the heap property (every parent node is greater than or equal to, or less than or equal to, its child nodes for max or min heaps respectively) and that it is almost completely filled, with the nodes on the last level as far left as possible. These checks can be accomplished through a combination of tree traversal techniques (such as level order traversal), comparisons of node values, and checking the completeness of the levels.

Step-by-step solution

Define the properties for heap

Understanding the heap property of the binary tree is crucial. In a max heap, each parent node is greater than or equal to its child node(s) and the key of the root node is the largest among other nodes. In a min heap, each parent node is less than or equal to its child node(s) and the key of the root node is the smallest among other nodes. The first part of the algorithm would be to test for these conditions.

Check for heapness

If the binary tree is a heap, it means it must satisfy the heap property. For each node in the tree, we check if this property holds. We traverse the tree from the root, comparing each parent node with its children. If the tree is a min-heap, every parent node should be less than or equal to its child nodes. If it's a max heap, every parent should be greater than or equal to its child nodes.

Check for completeness

An almost complete binary tree is a binary tree where all levels except possibly the last are completely filled, and in the last level, all the nodes are as far left as possible. So, the second part of the algorithm would be checking these conditions. We can do this by doing a level order traversal on the tree and checking if all the levels except possibly the last one are fully filled and in the last level, all the nodes are as left as possible.

Key concepts

Heap Property

Understanding the heap property is essential when dealing with binary trees, especially when implementing priority queues or sorting algorithms. The key characteristic of this property is the relationship between parent and child nodes. In a max heap, each parent node's value is greater than or equal to the values of its children, while in a min heap, the parent's value is less than or equal to its children's values.

This organization allows us to quickly extract the maximum or minimum value from the heap, as it will always be at the root. When validating whether a binary tree is a heap, we must ensure this property is consistent throughout the tree, starting from the root node down to the lowest-level nodes.

Binary Tree Traversal

To validate the heap property, it is necessary to traverse the binary tree. There are several approaches, such as in-order, pre-order, and post-order traversal. However, for heap validation, we typically use level order traversal.

Doing so allows us to inspect the nodes level by level, which aligns with the concept of ensuring that the heap property holds true for parent-child relationships, as children of a node will always appear in the same or next level of the tree. It's also the foundation for the method used when checking for the completeness of the tree.

Algorithm Design

Creating an algorithm for validating a binary tree as a heap involves a systematic approach. The algorithm should first define what it means for a tree to be a heap and then detect deviations from this definition.

In our case, the algorithm would start by checking the heap property of each node, working its way through the tree via traversal. If all nodes satisfy the heap property, the algorithm would move on to check for completeness, an additional requirement for a tree to qualify as a heap. Clear and logical steps, from checking heap property to completeness, ensure the reliability and efficiency of the algorithm.

Complete Binary Tree

A complete binary tree is a particular configuration of a binary tree where all levels are fully filled, with the possible exception of the bottom level, which must be filled from left to right. This structure ensures that the tree is as compact as possible.

It's important because a heap must also be a complete binary tree to ensure the best possible performance for operations like insertion and extraction. When verifying if a tree is a heap, one of the algorithm's steps will involve checking whether the tree satisfies the completeness property, following its structure closely.

Level Order Traversal

The level order traversal is a method of binary tree traversal that visits nodes level by level, starting from the root. This approach is particularly useful when we want to check if a binary tree is a complete binary tree.

In the context of our algorithm for validating a heap, after checking the heap property for each node, level order traversal is used to ensure that all levels, except the last, are fully filled and the nodes at the last level are placed to the left. Level order traversal is also the basis for constructing a breadth-first search algorithm, which represents another perspective where this traversal method is advantageous.