Tower of Hanoi recursion java

Question

Here's my Java code for solving Tower of Hanoi using recursion:

/**here is a stack of N disks on the first of three poles (call
them A, B and C) and your job is to move the disks from pole A to pole B without
ever putting a larger disk on top of a smaller disk.*/ 

public class Hanoi {

    public static void main(String[] args) {
        playHanoi (2,"A","B","C");
    }

    //move n disks from position "from" to "to" via "other"
    private static void playHanoi(int n, String from , String other, String to) {
        if (n == 0)
            return;
        if (n > 0)
        playHanoi(n-1, from, to, other);
        System.out.printf("Move one disk from pole %s to pole %s \n ", from, to);
        playHanoi(n-1, other, from, to);
    }

}

Does the place I put the print method matter? Also, can I do it like this:

    playHanoi(n-1, from, to, other);
    playHanoi(n-1, other, from, to);
    System.out.printf("Move one disk from pole %s to pole %s \n ", from, to);

Answer 1

Solving Tower of Hanoy problem in this way, is nothing but defining the strategy of how you want to get the job done. And your code :

    playHanoi(n-1, from, to, other);
    System.out.printf("Move one disk from pole %s to pole %s \n ", from, to);
    playHanoi(n-1, other, from, to);

Basically defines your strategy to like bellow,

1. Move n-1 disks from "from" (source tower) to "other" (intermediary tower).
2. Then move n th disk from "from" (source tower) to "to" (destination tower).
3. Finally move n-1 disks from "other" (intermediary tower) to "to" (destination tower).

Your prinf basically does the 2 nd step.

Now if you write code like this:

    playHanoi(n-1, from, to, other);
    playHanoi(n-1, other, from, to);
    System.out.printf("Move one disk from pole %s to pole %s \n ", from, to);

Then you are basically doing:

1. Move n-1 disks from "from" (source tower) to "other" (intermediary tower).

2. Then move n-1 disks from "other" (intermediary tower) to "to" (destination tower).

3. Finally move n th disk from "from" (source tower) to "to" (destination tower).

   In this strategy, after doing the 2 nd step (moving all n-1 disks from "other" to "to"), 3 rd step becomes invalid(moving n th disk from "from" to "to")! Because in Tower of Hanoy you cant put a larger disk on a smaller one!

So choosing the second option(strategy) leads you to an invalid strategy, thats why you can not do that!

Answer 2

It does indeed matter. Anything after your recursion call will be executed after that recursion unwinds (and anything before it, before), so you might find your output is in a nonsensical order.

Keep in mind that the statement after the function call doesn't execute until the function returns.