Unique Substrings

“Unique Substrings” can mean several different problems, so ask whether it seeks the longest substring with no repeated characters, the count of all distinct substring values, or unique fixed-length windows. For the common longest-no-repeat version, maintain a sliding window and a map from character to last seen index. When a repeated character lies inside the current window, move left to one past its previous occurrence; update the best length after each right step. This is O(n) time and O(alphabet) or O(n) map space. Counting all distinct substrings is fundamentally different: a suffix automaton or suffix array is appropriate for large inputs, while hashing every bounded-length substring needs collision care. Test empty strings, repeated characters, Unicode assumptions, and repeats outside the active window. State the definition before selecting complexity.