Add ReverseNumber implementation with tests

Author: gunkim

algorithm/src/main/java/io/github/gunkim/algorithm/basic/ReverseNumber.java

@@ -0,0 +1,23 @@
+package io.github.gunkim.algorithm.basic;
+
+public class ReverseNumber {
+    private final int value;
+
+    public ReverseNumber(int value) {
+        this.value = value;
+    }
+
+    public int reverse() {
+        if (value == 0) {
+            return value;
+        }
+        int number = Math.abs(value);
+        int reversed = 0;
+        while (number > 0) {
+            reversed = (reversed * 10) + (number % 10);
+            number /= 10;
+        }
+
+        return value < 0 ? -reversed : reversed;
+    }
+}

algorithm/src/test/java/io/github/gunkim/algorithm/basic/ReverseNumberTest.java

@@ -0,0 +1,41 @@
+package io.github.gunkim.algorithm.basic;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@DisplayName("숫자 뒤집기")
+class ReverseNumberTest {
+    @Test
+    void testPositiveNumber() {
+        var reverseNumber = new ReverseNumber(123);
+        int result = reverseNumber.reverse();
+
+        assertThat(result).isEqualTo(321);
+    }
+    
+    @Test
+    void testNegativeNumber() {
+        var reverseNumber = new ReverseNumber(-456);
+        int result = reverseNumber.reverse();
+    
+        assertThat(result).isEqualTo(-654);
+    }
+    
+    @Test
+    void testSingleDigit() {
+        var reverseNumber = new ReverseNumber(7);
+        int result = reverseNumber.reverse();
+    
+        assertThat(result).isEqualTo(7);
+    }
+    
+    @Test
+    void testZero() {
+        var reverseNumber = new ReverseNumber(0);
+        int result = reverseNumber.reverse();
+    
+        assertThat(result).isEqualTo(0);
+    }
+}