Pattern visualizer

Grow-and-shrink window

A repeat only forces the window to shrink, never to restart from scratch: when s[r] already lives in the window, evicting from the left until that clash clears is enough, because every character removed along the way would only re-trigger the same clash if it stayed. That's why both edges move forward only — r keeps probing new characters while l creeps right just enough to resolve a duplicate — so the window [l..r] never backtracks and each character is added and removed at most once. Reach for it when the answer is a contiguous run and validity can be checked incrementally, like "no repeated characters". Animated on: Longest substring without repeating characters in "abcabcbb".