Counting Sort

Question 1

When is Counting Sort most efficient?

Accepted Answer

When k (range) ≈ n or k << n — Counting Sort is efficient when the range of values (k) is comparable to or smaller than n.

Question 2

What is counting sort's time complexity for n elements with keys in range [0, k]?

Accepted Answer

O(n + k) — It counts occurrences in O(n), builds prefix positions in O(k), and places elements in O(n): total O(n + k) — linear when k = O(n).

Question 3

Counting sort beats the Ω(n log n) comparison lower bound because it…

Accepted Answer

It is not a comparison sort — The Ω(n log n) bound applies only to comparison sorts. Counting sort uses keys directly as array indices, sidestepping that bound.

Question 4

What is counting sort's space complexity?

Accepted Answer

A space cost of O(n + k) — It needs a count array of size k and an output array of size n, so O(n + k) auxiliary space — it is not in place.

Question 5

Counting sort is STABLE if elements are placed by…

Accepted Answer

Iterating backward via cumulative count — Iterating the input from the end and decrementing each key's cumulative count preserves the original relative order of equal keys — required for use as a radix-sort subroutine.

Question 6

Counting sort is a poor choice when…

Accepted Answer

When the key range is huge — Space and time scale with k. If k ≫ n (huge or sparse key space), the count array is enormous and wasteful — radix or comparison sorts are better.

Question 7

The role of the CUMULATIVE (prefix) sum over the count array is to…

Accepted Answer

Mark each key's output position — Prefix-summing the counts converts 'how many of each value' into 'where each value's block ends', giving each element its exact output position.

Question 8

To sort integers that include NEGATIVE values with counting sort, you…

Accepted Answer

Offset all keys by −min — Shift every key by subtracting the minimum (adding |min|) so the smallest maps to index 0; subtract the offset back when reading out.

Question 9

Counting sort is most useful as a building block for which algorithm?

Accepted Answer

Radix sort — Radix sort applies a stable counting sort on each digit/byte; counting sort's stability and O(n+k) per pass are exactly what radix needs.

Question 10

Why must counting sort's subroutine be STABLE for radix sort to work?

Accepted Answer

Prior digit order is preserved — LSD radix sort relies on each digit pass keeping the relative order established by previous passes; an unstable sort would scramble that, breaking correctness.

Counting Sort

Sort without comparing

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When is Counting Sort most efficient?

The short version

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Three Phases

Stability

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