# Towers of Hanoi implementation using stack.

The Tower of Hanoi is a mathematical game or puzzle. It consists of three rods, and a number of disks of different sizes which can slide onto any rod. The puzzle starts with the disks in a neat stack in ascending order of size on one rod, the smallest at the top, thus making a conical shape. The objective of the puzzle is to move the entire stack to another rod, obeying the following rules:

1) Only one disk must be moved at a time.

2) Each move consists of taking the upper disk from one of the rods and sliding it onto another rod, on top of the other disks that may already be present on that rod.

3) No disk may be placed on top of a smaller disk.

In this example, we are solving it by using stacks.

 ```package com.java2novice.ds.stack; public class TowersOfHanoiImpl { private static MyDynamicStack[] tower; public static void towersOfHanoi(int n) { // create three stacks, tower[0] is scratch tower = new MyDynamicStack[4]; for (int i = 0; i <= 3; i++){ tower[i] = new MyDynamicStack(4); } for (int d = n; d > 0; d--){ // initialize // add disk d to tower 1 tower[1].push(new Integer(d)); } // move n disks from tower 1 to 2 using 3 as // intermediate tower showTowerStates(n, 1, 2, 3); } public static void showTowerStates(int n, int x, int y, int z) { if (n > 0) { try{ showTowerStates(n - 1, x, z, y); // move d from top of tower x Integer d = (Integer) tower[x].pop(); // to top of tower y tower[y].push(d); System.out.println("Move disk " + d + " from tower "+ x + " to top of tower " + y); showTowerStates(n - 1, z, y, x); } catch(Exception ex){} } } public static void main(String[] args) { System.out.println("Running 3 disk problem:"); towersOfHanoi(3); } } public class MyDynamicStack { private int stackSize; private int[] stackArr; private int top; /** * constructor to create stack with size * @param size */ public MyDynamicStack(int size) { this.stackSize = size; this.stackArr = new int[stackSize]; this.top = -1; } /** * This method adds new entry to the top * of the stack * @param entry * @throws Exception */ public void push(int entry){ if(this.isStackFull()){ System.out.println(("Stack is full. Increasing the capacity.")); this.increaseStackCapacity(); } this.stackArr[++top] = entry; } /** * This method removes an entry from the * top of the stack. * @return * @throws Exception */ public int pop() throws Exception { if(this.isStackEmpty()){ throw new Exception("Stack is empty. Can not remove element."); } int entry = this.stackArr[top--]; return entry; } /** * This method returns top of the stack * without removing it. * @return */ public long peek() { return stackArr[top]; } private void increaseStackCapacity(){ int[] newStack = new int[this.stackSize*2]; for(int i=0;i

 Output: ```Running 3 disk problem: Move disk 1 from tower 1 to top of tower 2 Move disk 2 from tower 1 to top of tower 3 Move disk 1 from tower 2 to top of tower 3 Move disk 3 from tower 1 to top of tower 2 Move disk 1 from tower 3 to top of tower 1 Move disk 2 from tower 3 to top of tower 2 Move disk 1 from tower 1 to top of tower 2 ```

#### List of Stack Data Structure Examples

Knowledge Centre
What is abstract class or abstract method?
We cannot create instance for an abstract class. We can able to create instance for its subclass only. By specifying abstract keyword just before class, we can make a class as abstract class.

public abstract class MyAbstractClass{

}

Abstract class may or may not contains abstract methods. Abstract method is just method signature, it does not containes any implementation. Its subclass must provide implementation for abstract methods. Abstract methods are looks like as given below:

public abstract int getLength();
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