Added MinimumWindowSubstring algorithm and test (Sliding Window)

Author: jaimin45-art

src/main/java/com/thealgorithms/strings/MinimumWindowSubstring.java

@@ -0,0 +1,64 @@
+package com.thealgorithms.strings;
+
+/**
+ * Minimum Window Substring
+ *
+ * Given two strings s and t, return the minimum window substring of s such that
+ * every character in t (including duplicates) is included in the window.
+ *
+ * If there is no such substring, return an empty string "".
+ *
+ * Approach: Sliding Window + Frequency Table.
+ *
+ * Time complexity: O(n + m)
+ * Space complexity: O(1) for ASCII characters.
+ *
+ * Reference: https://en.wikipedia.org/wiki/Minimum_window_substring
+ */
+public final class MinimumWindowSubstring {
+
+    private MinimumWindowSubstring() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    public static String minWindow(String s, String t) {
+        if (s == null || t == null || s.length() < t.length() || t.isEmpty()) {
+            return "";
+        }
+
+        int[] need = new int[256];
+        for (char c : t.toCharArray()) {
+            need[c]++;
+        }
+
+        int required = 0;
+        for (int i = 0; i < 256; i++) {
+            if (need[i] > 0) required++;
+        }
+
+        int l = 0, r = 0, formed = 0;
+        int[] window = new int[256];
+        int minLen = Integer.MAX_VALUE, minLeft = 0;
+
+        while (r < s.length()) {
+            char c = s.charAt(r);
+            window[c]++;
+            if (need[c] > 0 && window[c] == need[c]) formed++;
+
+            while (l <= r && formed == required) {
+                if (r - l + 1 < minLen) {
+                    minLen = r - l + 1;
+                    minLeft = l;
+                }
+
+                char d = s.charAt(l);
+                window[d]--;
+                if (need[d] > 0 && window[d] < need[d]) formed--;
+                l++;
+            }
+            r++;
+        }
+
+        return minLen == Integer.MAX_VALUE ? "" : s.substring(minLeft, minLeft + minLen);
+    }
+}

src/test/java/com/thealgorithms/strings/MinimumWindowSubstringTest.java

@@ -0,0 +1,27 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import org.junit.jupiter.api.Test;
+
+public class MinimumWindowSubstringTest {
+
+    @Test
+    public void testExample1() {
+        assertEquals("BANC", MinimumWindowSubstring.minWindow("ADOBECODEBANC", "ABC"));
+    }
+
+    @Test
+    public void testExample2() {
+        assertEquals("", MinimumWindowSubstring.minWindow("A", "AA"));
+    }
+
+    @Test
+    public void testExample3() {
+        assertEquals("a", MinimumWindowSubstring.minWindow("a", "a"));
+    }
+
+    @Test
+    public void testExample4() {
+        assertEquals("t stri", MinimumWindowSubstring.minWindow("test string", "tist"));
+    }
+}