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Classic sliding window but I fumbled the shrinking logic at first.
Use a sliding window (two-pointer) technique to find the shortest substring containing all required characters. Expand the right pointer to include characters until all requirements are met, then shrink the left pointer to minimize the window while maintaining validity. Track the minimum length and return -1 if no valid window is found.
Pro tip: Clarify upfront whether the required characters must be distinct and whether the substring must contain at least one of each, not necessarily in order. This shows attention to detail and avoids incorrect assumptions.
Confirm that the set of required characters is distinct and that we need at least one occurrence of each. Discuss edge cases: empty string, empty set, or required characters not present in the string.
Explain that a brute-force check of all substrings is O(n^2) or worse, so a sliding window with two pointers achieves O(n) time by maintaining a window that can expand and contract.
Use a frequency map (or array) to count occurrences of required characters in the current window. Track how many required characters have been satisfied (count >= 1). A window is valid when all required characters are satisfied.
Move the right pointer to include new characters, updating counts and satisfaction. When the window is valid, move the left pointer to shrink it while it remains valid, updating the minimum length each time.
After scanning, return the minimum length found, or -1 if no valid window exists. State that time complexity is O(n) and space complexity is O(k) where k is the number of required characters.
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