Shortest Covering Substring for Subsequence

You are given two strings: a source string and a pattern string. Your task is to find the shortest contiguous substring within source such that the pattern is a subsequence of that substring. A subsequence is derived from another string by deleting some or no characters without changing the relative order of the remaining characters.

If there exists no contiguous substring in source that contains pattern as a subsequence, return an empty string "". If multiple substrings of the same minimum length satisfy the condition, return the one that appears earliest (has the smallest starting index) in the source string.

Key Concepts:

• A substring is a contiguous sequence of characters within a string.
• A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements.
• The goal is to find the minimum-length window in source where pattern exists as a subsequence.

Understanding Through Visualization:

For source = "abcdebdde" and pattern = "bde":

• The substring "bcde" (indices 1-4) contains "bde" as a subsequence: bcd****e
• The substring "bdde" (indices 5-8) also contains "bde" as a subsequence: bdd****e
• Both have length 4, but "bcde" starts earlier, so we return "bcde".
• Note: "deb" is NOT valid because the characters must appear in the same order as in pattern.