I was solving problem C from the last testing round 386C - Diverse Substrings
The idea I got was to handle substrings starting at different indices separately:
   1. You will do a precomputation next[i][26] telling you the next occurrence of each of the lower case letters from index i.
   2. You start at index i and each time you jump to the unseen character (using the precomputed array) with the least next occurrence    say k and all substrings starting at i and ending between the last jump and the current jump has diversity of the size of seen    characters so far.
The implementation was direct 5706503 and it was accepted.
I was playing around to optimize it when I thought I shouldn't loop on all characters and check what has been seen or not but instead maintain an actual set of the unseen characters so far and loop over them exclusively. The implementation was direct too 5706533.
To my surprise, this code timed out. I still can't explain why, it's supposed to be doing less loop iterations and hence less comparisons.
Any help would be appreciated.