##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
When to shrink
You need to track which characters are in the current window. A hash set works well. When you add a character that's already in the set, the window has a repeat.
Shrink from the left until the repeat is gone. Why does this work? The window always contains a valid substring (no repeats). You're finding the longest such window.
