Recursion Chapter 17

17 What is recursion? Recursion occurs when a method calls itself, either directly or indirectly. Used to solve difficult, repetitive problems

17 Factorial Example N! (N Factorial) N! = N * (N – 1)! for N >= 0 and 0! = 1 5! = 5 * 4! or 5 * 4 * 3 * 2 * 1 * 1 = 120 4! = 4 * 3! or 4 * 3 * 2 * 1 * 1 = 24 3! = 3 * 2! or 3 * 2 * 1 * 1 = 6 2! = 2 * 1! or 2 * 1 * 1 = 2 1! = 1 * 0! or 1 * 1 = 1 0! = 1

17 Calculate Factorials Using Recursion The Java solution: private int doFactorial(int N) { if ( N == 0 ) { return 1; } return N * doFactorial(N – 1); }

17 Towers of Hanoi Three towers hold rings stacked in order from largest to smallest (bottom to top) Player can only move one ring at a time Larger rings cannot be stacked on top of smaller ring Player must move stack to another tower

17 The Towers of Hanoi Algorithm Classic problem can be solved with recursion. Move N–1 rings to storage tower Move Nth ring to destination tower Move N–1 rings to destination tower

17 Towers of Hanoi Solution Final graphical solution includes the files: TowerRing.java Tower.java TowerMove.java ThreeTowers.java TowersOfHanoi.java TowerPanel.java

17 Graphical Towers of Hanoi Applet in progress