Explain why the decision tree of any sorting algorithm on n elements must have at least n! leaf nodes.

Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
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**Transcription:**

Explain why the decision tree of any sorting algorithm on \( n \) elements must have at least \( n! \) leaf nodes.

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**Explanation for Educational Context:**

In the context of sorting algorithms, the decision tree is a conceptual tool used to understand the comparisons made by the algorithm to determine a sorted order. Each internal node of the tree represents a comparison between elements, and each leaf node represents a possible outcome (i.e., a sorted permutation).

For a complete sorting of \( n \) elements, there are \( n! \) possible permutations — each a different way to arrange the elements in order. To account for all possible input sequences and outcomes, the decision tree must have at least \( n! \) leaf nodes, where each leaf node corresponds to a unique permutation. This ensures that for any initial arrangement of the elements, the algorithm can lead to the correct sorted order.

This concept underscores the lower bound of the complexity of comparison-based sorting algorithms, which is \( O(n \log n) \), as the structure of the decision tree implies a minimum number of comparisons required.
Transcribed Image Text:**Transcription:** Explain why the decision tree of any sorting algorithm on \( n \) elements must have at least \( n! \) leaf nodes. --- **Explanation for Educational Context:** In the context of sorting algorithms, the decision tree is a conceptual tool used to understand the comparisons made by the algorithm to determine a sorted order. Each internal node of the tree represents a comparison between elements, and each leaf node represents a possible outcome (i.e., a sorted permutation). For a complete sorting of \( n \) elements, there are \( n! \) possible permutations — each a different way to arrange the elements in order. To account for all possible input sequences and outcomes, the decision tree must have at least \( n! \) leaf nodes, where each leaf node corresponds to a unique permutation. This ensures that for any initial arrangement of the elements, the algorithm can lead to the correct sorted order. This concept underscores the lower bound of the complexity of comparison-based sorting algorithms, which is \( O(n \log n) \), as the structure of the decision tree implies a minimum number of comparisons required.
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The decision tree for any sorting algorithm must have n! permutations on n elements to sort properly.

 

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