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/main/java/com/thealgorithms/maths/ComplexNumberMultiply.java

@@ -0,0 +1,32 @@
+package com.thealgorithms.maths;
+
+/**
+ * Multiplies two complex numbers represented as strings in the form "a+bi".
+ * Supports negative values and validates input format.
+ */
+public final class ComplexNumberMultiply {
+
+    private ComplexNumberMultiply() {
+    }
+
+    private static int[] parse(String num) {
+        if (num == null || !num.matches("-?\\d+\\+-?\\d+i")) {
+            throw new IllegalArgumentException("Invalid complex number format: " + num);
+        }
+
+        String[] parts = num.split("\\+");
+        int real = Integer.parseInt(parts[0]);
+        int imaginary = Integer.parseInt(parts[1].replace("i", ""));
+        return new int[] {real, imaginary};
+    }
+
+    public static String multiply(String num1, String num2) {
+        int[] a = parse(num1);
+        int[] b = parse(num2);
+
+        int real = a[0] * b[0] - a[1] * b[1];
+        int imaginary = a[0] * b[1] + a[1] * b[0];
+
+        return real + "+" + imaginary + "i";
+    }
+}

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

@@ -0,0 +1,31 @@
+package com.thealgorithms.strings;
+
+/**
+ * Removes characters affected by '*' in a string.
+ * Each '*' deletes the closest non-star character to its left.
+ *
+ * Example:
+ * Input: leet**cod*e
+ * Output: lecoe
+ */
+
+public final class RemoveStars {
+
+    private RemoveStars() {
+    }
+
+    public static String removeStars(String s) {
+        StringBuilder result = new StringBuilder();
+
+        for (char c : s.toCharArray()) {
+            if (c == '*') {
+                if (result.length() > 0) {
+                    result.deleteCharAt(result.length() - 1);
+                }
+            } else {
+                result.append(c);
+            }
+        }
+        return result.toString();
+    }
+}

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*"));
+    }
+}