Question

I am having a problem understanding this Tower of Hanoi recursion algorithm:

public class MainClass {
  public static void main(String[] args) {
    int nDisks = 3;
    doTowers(nDisks, 'A', 'B', 'C');
  }

  public static void doTowers(int topN, char from, char inter, char to) {
    if (topN == 1){
      System.out.println("Disk 1 from " + from + " to " + to);
    }else {
      doTowers(topN - 1, from, to, inter);
      System.out.println("Disk " + topN + " from " + from + " to " + to);
      doTowers(topN - 1, inter, from, to);
    }
  }
}

The output is:

Disk 1 from A to C
Disk 2 from A to B
Disk 1 from C to B
Disk 3 from A to C
Disk 1 from B to A
Disk 2 from B to C
Disk 1 from A to C

I don’t understand how do we get:

Disk 1 from C to B
Disk 3 from A to C
Disk 1 from B to A

Can someone please explain?

Thank you.

Answer 1

Moving a N disks tower from peg A to C is achieved by moving a N-1 (all disks but the last one) tower from A to B, then moving the Nth disk from A to C, and finally moving the tower previously moved to B, from B to C. This shall be applied any time a tower with more than one disk is moved, in the case of a 1 disk tower, you just move its only disk.