Minimum Window Substring (LC #76) is the hard version of sliding window. Find the smallest substring containing all characters of a target string. Use two frequency maps: one for the target, one for the current window. Expand right until all target characters are covered. Then shrink from left while maintaining coverage, updating the minimum window. Track "characters satisfied" count to avoid comparing full maps each time. This runs in time and space where is the alphabet size.
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$ curl repovive.com/roadmaps/meta-interview-prep/arrays-strings/minimum-window-substring
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