Add StrobogrammaticNumber (#4278)

src/main/java/com/thealgorithms/maths/StrobogrammaticNumber.java

@@ -0,0 +1,43 @@
+package com.thealgorithms.maths;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * A strobogrammatic number is a number that remains the same when rotated 180 degrees.
+ * In other words, the number looks the same when rotated upside down.
+ * Examples of strobogrammatic numbers are "69", "88", "818", and "101".
+ * Numbers like "609" or "120" are not strobogrammatic because they do not look the same when rotated.
+ */
+public class StrobogrammaticNumber {
+    /**
+     * Check if a number is strobogrammatic
+     * @param number the number to be checked
+     * @return true if the number is strobogrammatic, false otherwise
+     */
+    public boolean isStrobogrammatic(String number) {
+        Map<Character, Character> strobogrammaticMap = new HashMap<>();
+        strobogrammaticMap.put('0', '0');
+        strobogrammaticMap.put('1', '1');
+        strobogrammaticMap.put('6', '9');
+        strobogrammaticMap.put('8', '8');
+        strobogrammaticMap.put('9', '6');
+
+        int left = 0;
+        int right = number.length() - 1;
+
+        while (left <= right) {
+            char leftChar = number.charAt(left);
+            char rightChar = number.charAt(right);
+
+            if (!strobogrammaticMap.containsKey(leftChar) || strobogrammaticMap.get(leftChar) != rightChar) {
+                return false;
+            }
+
+            left++;
+            right--;
+        }
+
+        return true;
+    }
+}

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

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+package com.thealgorithms.maths;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class StrobogrammaticNumberTest {
+
+    @Test
+    void testIsStrobogrammatic() {
+        StrobogrammaticNumber strobogrammaticNumber = new StrobogrammaticNumber();
+        assertThat(strobogrammaticNumber.isStrobogrammatic("69")).isTrue();
+        assertThat(strobogrammaticNumber.isStrobogrammatic("88")).isTrue();
+        assertThat(strobogrammaticNumber.isStrobogrammatic("818")).isTrue();
+        assertThat(strobogrammaticNumber.isStrobogrammatic("101")).isTrue();
+        assertThat(strobogrammaticNumber.isStrobogrammatic("609")).isTrue();
+        assertThat(strobogrammaticNumber.isStrobogrammatic("120")).isFalse();
+    }
+}