Given a string of a million numbers, return all repeating 3 digit numbers

Question

I had an interview with a hedge fund company in New York a few months ago and unfortunately, I did not get the internship offer as a data/software engineer. (They also asked

Answer 1

Constant time isn't possible. All 1 million digits need to be looked at at least once, so that is a time complexity of O(n), where n = 1 million in this case.

For a simple O(n) solution, create an array of size 1000 that represents the number of occurrences of each possible 3 digit number. Advance 1 digit at a time, first index == 0, last index == 999997, and increment array[3 digit number] to create a histogram (count of occurrences for each possible 3 digit number). Then output the content of the array with counts > 1.