Add Minimum Window Substring algorithm with JUnit tests in Sliding Window package #6552
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Description
This PR adds an implementation of the Minimum Window Substring problem and its corresponding unit tests under the com.thealgorithms.slidingwindow package.
The algorithm finds the smallest substring in s that contains all characters of t.
Time Complexity: O(n) (where n = length of s)
Space Complexity: O(|charset|) (for frequency hash maps)
Code Summary
Added MinimumWindowSubstring.java
Uses two hash maps (tFreq, windowFreq) and two pointers (left, right) to maintain the sliding window.
Returns the smallest valid substring containing all characters of t, or "" if no window exists.
Added MinimumWindowSubstringTest.java
Includes JUnit tests to validate correctness.
Covers multiple scenarios: basic matches, exact matches, and no-solution cases.