Solution: Minimum Window Subsequence

Statement

Given two strings, str1 and str2, find the shortest substring in str1 such that str2 is a subsequence of that substring.

A substring is defined as a contiguous sequence of characters within a string. A subsequence is a sequence that can be derived from another sequence by deleting zero or more elements without changing the order of the remaining elements.

Let’s say you have the following two strings:

str1 = “$abbcb$”

str2 = “$ac$”

In this example, “$abbc$” is a substring of str1, from which we can derive str2 simply by deleting both the instances of the character $b$. Therefore, str2 is a subsequence of this substring. Since this substring is the shortest among all the substrings in which str2 is present as a subsequence, the function should return this substring, that is, “$abbc$”.

If there is no substring in str1 that covers all characters in str2, return an empty string.

If there are multiple minimum-length substrings that meet the subsequence requirement, return the one with the left-most starting index.

Constraints:

• $1$ $\leq$ str1.length $\leq$ $2 \times 10^3$
• $1$ $\leq$ str2.length $\leq$ $100$
• str1 and str2 consist of uppercase and lowercase English letters.