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

For every letter keep track of the recent-most sighting. Whenever you process a letter, update the corresponding sighting index and calculate the range (max-min) of sighting indexes over all letters. Find the location with minimum range.

Complexity O(n). O(nlog(m)) if you consider alphabet size m.