Is there an O(n) integer sorting algorithm?

Yes, Radix Sort and Counting Sort are O(N). They are NOT comparison-based sorts, which have been proven to have Ω(N log N) lower bound.

To be precise, Radix Sort is O(kN), where k is the number of digits in the values to be sorted. Counting Sort is O(N + k), where k is the range of the numbers to be sorted.

There are specific applications where k is small enough that both Radix Sort and Counting Sort exhibit linear-time performance in practice.

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