Interview puzzle: Sorting a million number input with limited memory

Question

I tried answering this using external sort, but interviewer replied that the complexity was to high n.n(log(n)) i.e. n square *logn. Is there a better alternative.

T

Answer 1

Perhaps the interviewer expected you to ask: Are those numbers the unique seven digit telephone numbers mentioned by J. Bentley (Cracking the Oyster)?