An efficient algorithm for finding smallest pangrammatic windows?

Question

A pangrammatic window is a substring of a larger piece of text that contains all 26 letters of the alphabet. To quote an example from Wikipedia, given this

Answer 1

This algorithm has O(M) space complexity and O(N) time complexity (time does not depend on alphabet size M):

1. Advance first iterator and increase counter for each processed letter. Stop when all 26 counters are non-zero.
2. Advance second iterator and decrease counter for each processed letter. Stop when any of these counters is zero.
3. Use difference between iterators to update best-so-far result and continue with step 1.

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This algorithm may be improved a little bit if instead of character counters, positions in the string are stored. In this case step 2 should only read these positions and compare with current position, and step 1 should update these positions and (most of the time) search for some character in the text.