Add Collatz Conjecture (#3290)

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

@@ -0,0 +1,42 @@
+package com.thealgorithms.maths;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * <a href="https://en.wikipedia.org/wiki/Collatz_conjecture">...</a>
+ */
+public class CollatzConjecture {
+
+    /**
+     * Calculate the next number of the sequence.
+     *
+     * @param n current number of the sequence
+     * @return next number of the sequence
+     */
+    public int nextNumber(final int n) {
+        if (n % 2 == 0) {
+            return n / 2;
+        }
+        return 3 * n + 1;
+    }
+
+    /**
+     * Calculate the Collatz sequence of any natural number.
+     *
+     * @param firstNumber starting number of the sequence
+     * @return sequence of the Collatz Conjecture
+     */
+    public List<Integer> collatzConjecture(int firstNumber) {
+        if (firstNumber < 1) {
+            throw new IllegalArgumentException("Must be a natural number");
+        }
+        ArrayList<Integer> result = new ArrayList<>();
+        result.add(firstNumber);
+        while (firstNumber != 1) {
+            result.add(nextNumber(firstNumber));
+            firstNumber = nextNumber(firstNumber);
+        }
+        return result;
+    }
+}