Counting unique element in large array

Question

One of my colleagues was asked this question in an interview.

Given a huge array which stores unsigned int. Length of array is 100000000. Find the effective

Answer 1

Many other posters have suggested sorting the data and then finding the number of adjacent values, but no one has mentioned using radix sort yet to get the runtime to be O(n lg U) (where U is the maximum value in the array) instead of O(n lg n). Since lg U = O(lg n), assuming that integers take up one machine word, this approach is asymptotically faster than heapsort.

Non-comparison sorts are always fun in interviews. :-)