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

Heap sort is O(nlogn) and in-place. In-place is necessary when dealing with large data sets. Once sorted you can make one pass through the array tallying occurrences of each value. Because the array is sorted, once a value changes you know you've seen all occurrences of the previous value.