Add basic recursion algorithms

Author: usercodder99

src/main/java/com/thealgorithms/recursion/Factorial.java

@@ -0,0 +1,11 @@
+package com.thealgorithms.recursion;
+
+public class Factorial {
+
+    public static int factorial(int n) {
+        if (n == 0) {
+            return 1;
+        }
+        return n * factorial(n - 1);
+    }
+}

src/main/java/com/thealgorithms/recursion/Fibonacci.java

@@ -0,0 +1,11 @@
+package com.thealgorithms.recursion;
+
+public class Fibonacci {
+
+    public static int fibonacci(int n) {
+        if (n <= 1) {
+            return n;
+        }
+        return fibonacci(n - 1) + fibonacci(n - 2);
+    }
+}

src/main/java/com/thealgorithms/recursion/Permutations.java

@@ -0,0 +1,17 @@
+package com.thealgorithms.recursion;
+
+import java.util.ArrayList;
+
+public class Permutations {
+
+    public static void permute(String str, String prefix, ArrayList<String> result) {
+        if (str.length() == 0) {
+            result.add(prefix);
+        } else {
+            for (int i = 0; i < str.length(); i++) {
+                String rem = str.substring(0, i) + str.substring(i + 1);
+                permute(rem, prefix + str.charAt(i), result);
+            }
+        }
+    }
+}

src/main/java/com/thealgorithms/recursion/Subsets.java

@@ -0,0 +1,15 @@
+package com.thealgorithms.recursion;
+
+import java.util.ArrayList;
+
+public class Subsets {
+
+    public static void subset(String prefix, String str, ArrayList<String> result) {
+        if (str.length() == 0) {
+            result.add(prefix);
+            return;
+        }
+        subset(prefix + str.charAt(0), str.substring(1), result);
+        subset(prefix, str.substring(1), result);
+    }
+}

src/main/java/com/thealgorithms/recursion/TowerOfHanoi.java

@@ -0,0 +1,14 @@
+package com.thealgorithms.recursion;
+
+public class TowerOfHanoi {
+
+    public static void solve(int n, char from, char to, char aux) {
+        if (n == 1) {
+            System.out.println("Move disk 1 from rod " + from + " to rod " + to);
+            return;
+        }
+        solve(n - 1, from, aux, to);
+        System.out.println("Move disk " + n + " from rod " + from + " to rod " + to);
+        solve(n - 1, aux, to, from);
+    }
+}