feat: add needleman–wunsch sequence alignment algorithm (#6707)

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

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

@@ -0,0 +1,60 @@
+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 NeedlemanWunsch} class
+ */
+class NeedlemanWunschTest {
+
+    @Test
+    void testIdenticalStrings() {
+        int score = NeedlemanWunsch.align("GATTACA", "GATTACA", 1, -1, -2);
+        assertEquals(7, score); // All matches, 7*1
+    }
+
+    @Test
+    void testSimpleMismatch() {
+        int score = NeedlemanWunsch.align("GATTACA", "GACTATA", 1, -1, -2);
+        assertEquals(3, score);
+    }
+
+    @Test
+    void testInsertion() {
+        int score = NeedlemanWunsch.align("GATTACA", "GATACA", 1, -1, -2);
+        // One deletion (gap penalty)
+        assertEquals(4, score);
+    }
+
+    @Test
+    void testEmptyStrings() {
+        assertEquals(0, NeedlemanWunsch.align("", "", 1, -1, -2));
+    }
+
+    @Test
+    void testOneEmpty() {
+        assertEquals(-14, NeedlemanWunsch.align("GATTACA", "", 1, -1, -2)); // 7 gaps × -2
+    }
+
+    @Test
+    void testGapHeavyAlignment() {
+        int score = NeedlemanWunsch.align("AAAA", "AA", 1, -1, -2);
+        assertEquals(-2, score); // Two matches (2*1) + two gaps (2*-2)
+    }
+
+    @ParameterizedTest
+    @CsvSource({"null,ABC", "ABC,null", "null,null"})
+    void testNullInputs(String s1, String s2) {
+        // Interpret "null" literal as Java null
+        String first = "null".equals(s1) ? null : s1;
+        String second = "null".equals(s2) ? null : s2;
+
+        IllegalArgumentException ex = assertThrows(IllegalArgumentException.class, () -> NeedlemanWunsch.align(first, second, 1, -1, -2));
+        assertEquals("Input strings must not be null.", ex.getMessage());
+    }
+}