Add string algorithms: RemoveStars and ComplexNumberMultiply (#7275)

* Add RemoveStars and ComplexNumberMultiply string algorithms

* Add RemoveStars and ComplexNumberMultiply string algorithms

* Add unit tests for RemoveStars and ComplexNumber Multiply

* Fix checkstyle

* Remove redundant main method

* Move ComplexNumberMultiply to maths package and add input validation with tests

* Apply spotless formatting

---------

Co-authored-by: Deniz Altunkapan <deniz.altunkapan@outlook.com>

src/test/java/com/thealgorithms/maths/ComplexNumberMultiplyTest.java

@@ -0,0 +1,34 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+public class ComplexNumberMultiplyTest {
+
+    @Test
+    void testExample() {
+        assertEquals("0+2i", ComplexNumberMultiply.multiply("1+1i", "1+1i"));
+    }
+
+    @Test
+    void testNegative() {
+        assertEquals("0+-2i", ComplexNumberMultiply.multiply("1+-1i", "1+-1i"));
+    }
+
+    @Test
+    void testZero() {
+        assertEquals("0+0i", ComplexNumberMultiply.multiply("0+0i", "5+3i"));
+    }
+
+    @Test
+    void testInvalidFormat() {
+        assertThrows(IllegalArgumentException.class, () -> ComplexNumberMultiply.multiply("1+1", "1+1i"));
+    }
+
+    @Test
+    void testNullInput() {
+        assertThrows(IllegalArgumentException.class, () -> ComplexNumberMultiply.multiply(null, "1+1i"));
+    }
+}

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

@@ -0,0 +1,28 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class RemoveStarsTest {
+
+    @Test
+    void testExampleCase() {
+        assertEquals("lecoe", RemoveStars.removeStars("leet**cod*e"));
+    }
+
+    @Test
+    void testAllStars() {
+        assertEquals("", RemoveStars.removeStars("abc***"));
+    }
+
+    @Test
+    void testNoStars() {
+        assertEquals("hello", RemoveStars.removeStars("hello"));
+    }
+
+    @Test
+    void testSingleCharacter() {
+        assertEquals("", RemoveStars.removeStars("a*"));
+    }
+}