The sliding window part I was fine with, two pointers, frequency maps, shrink from the left when valid.
Use a sliding window with two pointers to expand and contract the window while tracking character frequencies. Maintain a count of matched characters to know when the window contains all target characters, and record the minimum length. Then design test cases covering edge cases like empty strings, no solution, and duplicates.
Pro tip: Clarify upfront that the target may contain duplicate characters and that the window must contain at least the same count of each. Mention that the algorithm runs in O(n) time using a frequency map, which is optimal for this problem.
Confirm that the substring must contain all characters of the target including duplicates, and that if no such substring exists, return an empty string. Discuss edge cases like empty source or target, target longer than source, and characters not in source.
Use two pointers (left and right) to represent a window. Expand right to include characters until the window is valid (contains all target characters with required counts), then contract left to find the smallest valid window. Track the minimum length and starting index.
Use a hash map to count characters needed from the target. As you expand the window, decrement the needed count for each character. Maintain a 'formed' counter to track how many unique characters have met the required count. When formed equals the number of unique characters in target, the window is valid.
After finding a valid window, contract from the left while the window remains valid, updating the minimum length. If the minimum length is never updated, return an empty string. Ensure the algorithm handles cases where target has characters not in source.
Create test cases covering: normal case with duplicates, no valid substring, empty strings, target longer than source, and characters with varying frequencies. Include a case where the shortest substring is at the beginning or end.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.