From 9b9bfd7f52fef56120e3a53bfc3b0501c83805ad Mon Sep 17 00:00:00 2001
From: Rosander0 <astereen.0107@gmail.com>
Date: Wed, 10 Jun 2026 18:30:04 +0530
Subject: [PATCH] feat: add LongestCommonSubstring implementation

---
 pom.xml                                       |  2 +-
 .../strings/LongestCommonSubstring.java       | 55 +++++++++++++++++++
 .../strings/LongestCommonSubstringTest.java   | 36 ++++++++++++
 3 files changed, 92 insertions(+), 1 deletion(-)
 create mode 100644 src/main/java/com/thealgorithms/strings/LongestCommonSubstring.java
 create mode 100644 src/test/java/com/thealgorithms/strings/LongestCommonSubstringTest.java

diff --git a/pom.xml b/pom.xml
index 6edd36f446d8..e54b83399e9f 100644
--- a/pom.xml
+++ b/pom.xml
@@ -119,7 +119,7 @@
             <plugin>
                 <groupId>com.github.spotbugs</groupId>
                 <artifactId>spotbugs-maven-plugin</artifactId>
-                <version>4.9.8.3</version>
+                <version>4.10.2.0</version>
                 <configuration>
                     <excludeFilterFile>spotbugs-exclude.xml</excludeFilterFile>
                     <includeTests>true</includeTests>
diff --git a/src/main/java/com/thealgorithms/strings/LongestCommonSubstring.java b/src/main/java/com/thealgorithms/strings/LongestCommonSubstring.java
new file mode 100644
index 000000000000..b2190316aff2
--- /dev/null
+++ b/src/main/java/com/thealgorithms/strings/LongestCommonSubstring.java
@@ -0,0 +1,55 @@
+package com.thealgorithms.strings;
+
+/**
+ * Longest Common Substring finds the longest string that is a
+ * contiguous substring of two input strings.
+ * Example: "abcdef" and "zcdemf" -> "cde"
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Longest_common_substring">
+ *     Wikipedia: Longest Common Substring</a>
+ *
+ * author: Vraj Prajapati @Rosander0
+ */
+public final class LongestCommonSubstring {
+
+    private LongestCommonSubstring() {
+        // Utility class
+    }
+
+    /**
+     * Finds the longest common substring of two strings.
+     *
+     * @param a First input string
+     * @param b Second input string
+     * @return The longest common substring, or empty string if none exists.
+     *     If multiple substrings share the maximum length, the first one found is returned.
+     */
+    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)) {
+                    dp[i][j] = dp[i - 1][j - 1] + 1;
+                    if (dp[i][j] > maxLength) {
+                        maxLength = dp[i][j];
+                        endIndex = i;
+                    }
+                } else {
+                    dp[i][j] = 0;
+                }
+            }
+        }
+
+        if (maxLength == 0) {
+            return "";
+        }
+        return a.substring(endIndex - maxLength, endIndex);
+    }
+}
diff --git a/src/test/java/com/thealgorithms/strings/LongestCommonSubstringTest.java b/src/test/java/com/thealgorithms/strings/LongestCommonSubstringTest.java
new file mode 100644
index 000000000000..e54abcf2f1f3
--- /dev/null
+++ b/src/test/java/com/thealgorithms/strings/LongestCommonSubstringTest.java
@@ -0,0 +1,36 @@
+package com.thealgorithms.strings;
+// author: Vraj Prajapati @Rosander0
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class LongestCommonSubstringTest {
+
+    @Test
+    public void testNullOrEmptyInputs() {
+        assertEquals("", LongestCommonSubstring.longestCommonSubstring(null, "abc"));
+        assertEquals("", LongestCommonSubstring.longestCommonSubstring("abc", null));
+        assertEquals("", LongestCommonSubstring.longestCommonSubstring("", "abc"));
+        assertEquals("", LongestCommonSubstring.longestCommonSubstring("abc", ""));
+    }
+
+    @Test
+    public void testNormalSubstrings() {
+        assertEquals("cde", LongestCommonSubstring.longestCommonSubstring("abcdef", "zcdemf"));
+        assertEquals("abc", LongestCommonSubstring.longestCommonSubstring("abc", "abc"));
+        assertEquals("cdef", LongestCommonSubstring.longestCommonSubstring("abcdef", "cdefgh"));
+    }
+
+    @Test
+    public void testSingleCharacterAndNoMatch() {
+        assertEquals("a", LongestCommonSubstring.longestCommonSubstring("a", "a"));
+        assertEquals("", LongestCommonSubstring.longestCommonSubstring("abc", "xyz"));
+    }
+
+    @Test
+    public void testMultipleMatchesFirstLongest() {
+        // Keeps the first matched longest substring when lengths are tied
+        assertEquals("abc", LongestCommonSubstring.longestCommonSubstring("abcXdef", "abcYdef"));
+    }
+}