Problem

Finding the minimum substring of str1 that contains all the characters in str2.

Solution

The solution is to use a sliding window. We can keep a count of how many unique characters in str2 we have seen so far. If we have seen all the unique characters in str2, then we know we have found a substring of str1 that contains all the characters in str2. We can keep track of the minimum length substring by comparing it to the current substring. If the current substring is shorter, then we update the minimum length substring.