Towers Of Hanoi with Stacks Implementation and Recursion (Java)

Question

I'm trying to solve the Towers of Hanoi problem through recursion, while using stacks. The output should look as follows, using n # of disks of increasing size. Disks must move from Pillar1 to Pillar3, one at a time:

// assuming n = 3;

Pillar1: 3 2 1

Pillar2:

Pillar3: 

// "\n"

Pillar1: 3 2 

Pillar2:

Pillar3: 1

// "\n"

Pillar1: 3 

Pillar2: 2

Pillar3: 1


// "\n"

Pillar1: 3 

Pillar2: 2 1

Pillar3: 


// "\n"

Pillar1: 

Pillar2: 2 1

Pillar3: 3


// "\n"

Pillar1: 1

Pillar2: 2

Pillar3: 3

// "\n"

Pillar1: 1

Pillar2: 

Pillar3: 3 2


// "\n"

Pillar1: 

Pillar2: 

Pillar3: 3 2 1

My code is below, I am having a hard time with the output with disks > 1:

import java.util.*;
class TowersOfHanoiThree
{
   public static Stack<Integer>[] tower = new Stack[4];
   public static int temp;

   public static void TowersOfHanoiThree(int numDisk)
   {
      //adding disk to stack
      temp = numDisk;
      tower = new Stack[4];

      for(int a = 0; a <= 3; a++)
      {
         tower[a] = new Stack<Integer>();
      }

     for (int i = numDisk; i > 0; i--)
     {
        tower[1].push(numDisk);
        show();
     }
     solver(numDisk, 1, 3, 2);
 }

public static void show()
{
   //System.out.println("Pillar1: ");
   //System.out.println("Pillar2: ");
   //System.out.println("Pillar3: ");

   String Pillar1 = "Pillar1: ";
   String Pillar2 = "Pillar2: ";
   String Pillar3 = "Pillar3: ";

   for(int x = temp -1 ; x >= 0 ; x--)
   {
      String emptStr1 = "";
      String emptStr2 = "";
      String emptStr3 = "";

     try
     {
        emptStr1 = String.valueOf(tower[1].get(x));
     }
     catch(Exception e)
     {
     }

     try
     {
        emptStr2 = String.valueOf(tower[2].get(x));
     }
     catch(Exception e)
     {
     }

     try
     {
        emptStr3 = String.valueOf(tower[3].get(x));
     }
     catch(Exception e)
     {
     }
     System.out.print(Pillar1+emptStr1+"\n");
     System.out.print(Pillar2+emptStr2+"\n");
     System.out.print(Pillar3+emptStr3+"\n");
     System.out.print("\n");
  }
}//end show

public static void solver(int numDisk, int start, int middle, int end) 
{
   if(numDisk > 0) 
   {
      try
      {
         //sorting disks
         solver(numDisk - 1, start, end, middle);
         int dis = tower[start].pop(); //move disk top-most disk of start
         tower[middle].push(dis);
         show();
         solver(numDisk - 1, middle, start, end);
      }
      catch(Exception e)
      {
      }
   }
}

public static void main(String args[])
{
    tower[1] = new Stack<Integer>();
    tower[2] = new Stack<Integer>();
    tower[3] = new Stack<Integer>();

    TowersOfHanoiThree(2);
}
}

Answer 1

Using the following implementation should offer you the desired result. I have also placed some inline comments with the modifications made

public static void TowersOfHanoiThree(int numDisk)
{
  //adding disk to stack
  temp = numDisk;
  tower = new Stack[4];

  for(int a = 0; a <= 3; a++)
  {
     tower[a] = new Stack<Integer>();
  }

 for (int i = numDisk; i > 0; i--)
 {
    tower[1].push(i); // to show "1 2 3" i changed the value which was pushed in the stack
// from tower[1].push(numDisk) to tower[1].push(i)
 }
 show(); //In your example this method call was placed inside the for loop.
//Moving it outside will show the stacks only after the values are added
 solver(numDisk, 1, 3, 2);
}

 public static void show() {
    // System.out.println("Pillar1: ");
    // System.out.println("Pillar2: ");
    // System.out.println("Pillar3: ");

    String Pillar1 = "Pillar1: ";
    String Pillar2 = "Pillar2: ";
    String Pillar3 = "Pillar3: ";

    String emptStr1 = "";
    String emptStr2 = "";
    String emptStr3 = "";
//the empStr variable are moved outside the loop because only after the 
//loop has finished we know what values are in each pillar
    for (int x = 0; x <= temp - 1; x++) {

        try {
//here we just append the values from the pillar to string empStr1
            emptStr1 += String.valueOf(tower[1].get(x)) + " ";
        } catch (Exception e) {
        }

        try {
            emptStr2 += String.valueOf(tower[2].get(x)) + " ";
        } catch (Exception e) {
        }

        try {
            emptStr3 += String.valueOf(tower[3].get(x)) + " ";
        } catch (Exception e) {
        }
    }
//finally when the loop is done we have the results
    System.out.print(Pillar1 + emptStr1 + "\n");
    System.out.print(Pillar2 + emptStr2 + "\n");
    System.out.print(Pillar3 + emptStr3 + "\n");
    System.out.print("\n");
}// end show