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The algorithm you're studying is known as the Median of Medians algorithm for finding the k-th smallest element in linear time, or O(n). The algorithm relies on dividing the sequence S into groups of 5 because using groups of this size allows for an efficient selection of approximate medians, which are critical to the algorithm's efficiency.
If you try dividing into groups of 6, 7, or 10 elements instead of 5, it would affect the efficiency and accuracy of the median-finding step. Here's why the group size matters:
- Group of 5 Advantage: By choosing groups of 5, the algorithm ensures that the median of medians (the pivot) is close to the true median of the entire set, which enables efficient partitioning. Smaller groups (like 5) yield a pivot that's more likely to split the array into relatively balanced parts, which is essential for keeping the recursion depth low and maintaining linear performance.
- Effect of Larger Group Sizes: If you switch to groups of 6, 7, or 10, the pivot might no longer be as close to the true median, leading to imbalanced partitions. This imbalance increases the chance of needing more recursive steps, which can push the algorithm closer to O(nlogn) performance rather than O(n). The larger the group size, the less reliable the median of medians approach becomes for maintaining linear performance.
- Why Not Smaller Groups (Like 3): If you used groups smaller than 5, such as 3, the pivot would be less accurate, often resulting in even more imbalanced partitions, which would also harm the performance.
In summary, the choice of 5 is a balance between accurate pivot selection and manageable computational overhead. Changing the group size to 6, 7, or 10 would reduce the accuracy of the pivot and risk losing the algorithm's linear time complexity.
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