feat: add smith-waterman sequence alignment algorithm (#6708)

Co-authored-by: a <alexanderklmn@gmail.com>

src/test/java/com/thealgorithms/dynamicprogramming/SmithWatermanTest.java

@@ -0,0 +1,53 @@
+package com.thealgorithms.dynamicprogramming;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+/**
+ * Unit tests for the {@code SmithWaterman} class.
+ */
+class SmithWatermanTest {
+
+    @Test
+    void testIdenticalStrings() {
+        int score = SmithWaterman.align("GATTACA", "GATTACA", 2, -1, -2);
+        assertEquals(14, score); // full match, 7*2
+    }
+
+    @Test
+    void testPartialMatch() {
+        int score = SmithWaterman.align("GATTACA", "TTAC", 2, -1, -2);
+        assertEquals(8, score); // best local alignment "TTAC"
+    }
+
+    @Test
+    void testNoMatch() {
+        int score = SmithWaterman.align("AAAA", "TTTT", 1, -1, -2);
+        assertEquals(0, score); // no alignment worth keeping
+    }
+
+    @Test
+    void testInsertionDeletion() {
+        int score = SmithWaterman.align("ACGT", "ACGGT", 1, -1, -2);
+        assertEquals(3, score); // local alignment "ACG"
+    }
+
+    @Test
+    void testEmptyStrings() {
+        assertEquals(0, SmithWaterman.align("", "", 1, -1, -2));
+    }
+
+    @ParameterizedTest
+    @CsvSource({"null,ABC", "ABC,null", "null,null"})
+    void testNullInputs(String s1, String s2) {
+        String first = "null".equals(s1) ? null : s1;
+        String second = "null".equals(s2) ? null : s2;
+
+        IllegalArgumentException ex = assertThrows(IllegalArgumentException.class, () -> SmithWaterman.align(first, second, 1, -1, -2));
+        assertEquals("Input strings must not be null.", ex.getMessage());
+    }
+}