fix: resolve build errors and formatting

Author: Rosander0

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

@@ -1,9 +1,9 @@
 package com.thealgorithms.strings;
 
 /**
- * Longest Common Substring finds the longest string that is a 
+ * Longest Common Substring finds the longest string that is a
  * contiguous substring of two input strings.
- * Example: "abcdef" and "zcdemf" → "cde"
+ * Example: "abcdef" and "zcdemf" -> "cde"
  */
 public final class LongestCommonSubstring {
 
@@ -22,9 +22,11 @@ public static String longestCommonSubstring(final String a, final String b) {
         if (a == null || b == null || a.isEmpty() || b.isEmpty()) {
             return "";
         }
+        
         int[][] dp = new int[a.length() + 1][b.length() + 1];
         int maxLength = 0;
         int endIndex = 0;
+        
         for (int i = 1; i <= a.length(); i++) {
             for (int j = 1; j <= b.length(); j++) {
                 if (a.charAt(i - 1) == b.charAt(j - 1)) {
@@ -33,6 +35,8 @@ public static String longestCommonSubstring(final String a, final String b) {
                         maxLength = dp[i][j];
                         endIndex = i;
                     }
+                } else {
+                    dp[i][j] = 0;
                 }
             }
         }

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

@@ -1,16 +1,18 @@
 package com.thealgorithms.strings;
 
 import java.util.stream.Stream;
+
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.MethodSource;
 
-public class LongestCommonSubstringTest {
+public final class LongestCommonSubstringTest {
 
     private record TestData(String a, String b, String expected) {
     }
 
-    private static Stream<TestData> provideTestCases() {
+    // JUnit 5 matches this automatically because it has the same name as the test method
+    private static Stream<TestData> testLongestCommonSubstring() {
         return Stream.of(
             new TestData(null, "abc", ""),
             new TestData("abc", null, ""),
@@ -20,14 +22,14 @@ private static Stream<TestData> provideTestCases() {
             new TestData("abc", "abc", "abc"),
             new TestData("abc", "xyz", ""),
             new TestData("abcdef", "cdefgh", "cdef"),
-            new TestData("a", "a", "a")
+            new TestData("a", "a", "a"),
+            new TestData("abcXdef", "abcYdef", "abc")
         );
     }
 
     @ParameterizedTest
-    @MethodSource("provideTestCases")
+    @MethodSource // No string argument needed here anymore
     void testLongestCommonSubstring(TestData testData) {
-        Assertions.assertEquals(testData.expected(),
-            LongestCommonSubstring.longestCommonSubstring(testData.a(), testData.b()));
+        Assertions.assertEquals(testData.expected(), LongestCommonSubstring.longestCommonSubstring(testData.a(), testData.b()));
     }
 }