Added Resursive implementation to get nth fibonacci number with memoization

Author: pri-sin

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

@@ -0,0 +1,42 @@
+package com.thealgorithms.maths;
+
+import java.math.BigInteger;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * author : @pri-sin
+ * This class provides methods for calculating Fibonacci numbers Recursion.
+ */
+public final class FibonacciJavaRecursion {
+	static Map<Integer, BigInteger> fibonacciMap=new HashMap<>();
+
+    private FibonacciJavaRecursion() {
+        // Private constructor to prevent instantiation of this utility class.
+    }
+
+    /**
+     * Calculates the nth Fibonacci number.
+     *
+     * @param n The index of the Fibonacci number to calculate.
+     * @return The nth Fibonacci number as a BigInteger.
+     * @throws IllegalArgumentException if the input 'n' is a negative integer.
+     */
+    public static BigInteger computeRecursively(final int n) {
+        if (n < 0) {
+            throw new IllegalArgumentException("Input 'n' must be a non-negative integer.");
+        }
+        
+        if (n <= 1) {
+        	fibonacciMap.put(n, BigInteger.valueOf(n));
+            return BigInteger.valueOf(n);
+        }
+        
+        if(!fibonacciMap.containsKey(n)) {
+        	fibonacciMap.put(n, computeRecursively(n-2).add(computeRecursively(n-1)));
+        }
+
+        return fibonacciMap.get(n);
+    }
+    
+}

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

@@ -0,0 +1,36 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.math.BigInteger;
+import org.junit.jupiter.api.Test;
+
+public class FibonacciJavaRecursionTest {
+    @Test
+    public void checkValueAtZero() {
+        assertEquals(BigInteger.ZERO, FibonacciJavaRecursion.computeRecursively(0));
+    }
+
+    @Test
+    public void checkValueAtOne() {
+        assertEquals(BigInteger.ONE, FibonacciJavaRecursion.computeRecursively(1));
+    }
+
+    @Test
+    public void checkValueAtTwo() {
+        assertEquals(BigInteger.ONE, FibonacciJavaRecursion.computeRecursively(2));
+    }
+
+    @Test
+    public void checkRecurrenceRelation() {
+        for (int i = 0; i < 100; ++i) {
+            assertEquals(FibonacciJavaRecursion.computeRecursively(i + 2), FibonacciJavaRecursion.computeRecursively(i + 1).add(FibonacciJavaRecursion.computeRecursively(i)));
+        }
+    }
+
+    @Test
+    public void checkNegativeInput() {
+        assertThrows(IllegalArgumentException.class, () -> { FibonacciJavaRecursion.computeRecursively(-1); });
+    }
+}