Added MinimumWindowSubstring algorithm and test (Sliding Window)

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"));
+ }
+}