Iterative Tower of Hanoi

Difficulty Level : Hard
Last Updated : 28 Jun, 2021

The Tower of Hanoi is a mathematical puzzle. It consists of three poles and a number of disks of different sizes which can slide onto any poles. The puzzle starts with the disk in a neat stack in ascending order of size in one pole, the smallest at the top thus making a conical shape. The objective of the puzzle is to move all the disks from one pole (say ‘source pole’) to another pole (say ‘destination pole’) with the help of the third pole (say auxiliary pole).

The puzzle has the following two rules:
1. You can’t place a larger disk onto a smaller disk
2. Only one disk can be moved at a time

We’ve already discussed a recursive solution for the Tower of Hanoi. We have also seen that for n disks, a total of 2ⁿ – 1 moves are required.

Iterative Algorithm:

1. Calculate the total number of moves required i.e. "pow(2, n)
   - 1" here n is number of disks.
2. If number of disks (i.e. n) is even then interchange destination 
   pole and auxiliary pole.
3. for i = 1 to total number of moves:
     if i%3 == 1:
    legal movement of top disk between source pole and 
        destination pole
     if i%3 == 2:
    legal movement top disk between source pole and 
        auxiliary pole    
     if i%3 == 0:
        legal movement top disk between auxiliary pole 
        and destination pole

Example:

Let us understand with a simple example with 3 disks:
So, total number of moves required = 7

        S                      A                   D

When i= 1, (i % 3 == 1) legal movement between‘S’ and ‘D’

When i = 2,  (i % 3 == 2) legal movement between ‘S’ and ‘A’

When i = 3, (i % 3 == 0) legal movement between ‘A’ and ‘D’ ’

When i = 4, (i % 3 == 1) legal movement between ‘S’ and ‘D’

When i = 5, (i % 3 == 2) legal movement between ‘S’ and ‘A’

When i = 6, (i % 3 == 0) legal movement between ‘A’ and ‘D’

When i = 7, (i % 3 == 1) legal movement between ‘S’ and ‘D’

So, after all these destination poles contains all the in order of size.
After observing above iterations, we can think that after a disk other than the smallest disk is moved, the next disk to be moved must be the smallest disk because it is the top disk resting on the spare pole and there are no other choices to move a disk.

C++

// C Program for Iterative Tower of Hanoi
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <limits.h>
 
// A structure to represent a stack
struct Stack
{
unsigned capacity;
int top;
int *array;
};
 
// function to create a stack of given capacity.
struct Stack* createStack(unsigned capacity)
{
    struct Stack* stack =
        (struct Stack*) malloc(sizeof(struct Stack));
    stack -> capacity = capacity;
    stack -> top = -1;
    stack -> array =
        (int*) malloc(stack -> capacity * sizeof(int));
    return stack;
}
 
// Stack is full when top is equal to the last index
int isFull(struct Stack* stack)
{
return (stack->top == stack->capacity - 1);
}
 
// Stack is empty when top is equal to -1
int isEmpty(struct Stack* stack)
{
return (stack->top == -1);
}
 
// Function to add an item to stack. It increases
// top by 1
void push(struct Stack *stack, int item)
{
    if (isFull(stack))
        return;
    stack -> array[++stack -> top] = item;
}
 
// Function to remove an item from stack. It
// decreases top by 1
int pop(struct Stack* stack)
{
    if (isEmpty(stack))
        return INT_MIN;
    return stack -> array[stack -> top--];
}
 
//Function to show the movement of disks
void moveDisk(char fromPeg, char toPeg, int disk)
{
    printf("Move the disk %d from \'%c\' to \'%c\'\n",
        disk, fromPeg, toPeg);
}
 
// Function to implement legal movement between
// two poles
void moveDisksBetweenTwoPoles(struct Stack *src,
            struct Stack *dest, char s, char d)
{
    int pole1TopDisk = pop(src);
    int pole2TopDisk = pop(dest);
 
    // When pole 1 is empty
    if (pole1TopDisk == INT_MIN)
    {
        push(src, pole2TopDisk);
        moveDisk(d, s, pole2TopDisk);
    }
 
    // When pole2 pole is empty
    else if (pole2TopDisk == INT_MIN)
    {
        push(dest, pole1TopDisk);
        moveDisk(s, d, pole1TopDisk);
    }
 
    // When top disk of pole1 > top disk of pole2
    else if (pole1TopDisk > pole2TopDisk)
    {
        push(src, pole1TopDisk);
        push(src, pole2TopDisk);
        moveDisk(d, s, pole2TopDisk);
    }
 
    // When top disk of pole1 < top disk of pole2
    else
    {
        push(dest, pole2TopDisk);
        push(dest, pole1TopDisk);
        moveDisk(s, d, pole1TopDisk);
    }
}
 
//Function to implement TOH puzzle
void tohIterative(int num_of_disks, struct Stack
            *src, struct Stack *aux,
            struct Stack *dest)
{
    int i, total_num_of_moves;
    char s = 'S', d = 'D', a = 'A';
 
    //If number of disks is even, then interchange
    //destination pole and auxiliary pole
    if (num_of_disks % 2 == 0)
    {
        char temp = d;
        d = a;
        a = temp;
    }
    total_num_of_moves = pow(2, num_of_disks) - 1;
 
    //Larger disks will be pushed first
    for (i = num_of_disks; i >= 1; i--)
        push(src, i);
 
    for (i = 1; i <= total_num_of_moves; i++)
    {
        if (i % 3 == 1)
        moveDisksBetweenTwoPoles(src, dest, s, d);
 
        else if (i % 3 == 2)
        moveDisksBetweenTwoPoles(src, aux, s, a);
 
        else if (i % 3 == 0)
        moveDisksBetweenTwoPoles(aux, dest, a, d);
    }
}
 
// Driver Program
int main()
{
    // Input: number of disks
    unsigned num_of_disks = 3;
 
    struct Stack *src, *dest, *aux;
 
    // Create three stacks of size 'num_of_disks'
    // to hold the disks
    src = createStack(num_of_disks);
    aux = createStack(num_of_disks);
    dest = createStack(num_of_disks);
 
    tohIterative(num_of_disks, src, aux, dest);
    return 0;
}

Java

// Java program for iterative
// Tower of Hanoi
class TOH{
     
// A structure to represent a stack
class Stack
{
    int capacity;
    int top;
    int array[];
}
 
// Function to create a stack of given capacity.
Stack createStack(int capacity)
{
    Stack stack = new Stack();
    stack.capacity = capacity;
    stack.top = -1;
    stack.array = new int[capacity];
    return stack;
}
 
// Stack is full when the top is equal
// to the last index
boolean isFull(Stack stack)
{
    return (stack.top == stack.capacity - 1);
}
 
// Stack is empty when top is equal to -1
boolean isEmpty(Stack stack)
{
    return (stack.top == -1);
}
 
// Function to add an item to stack.It
// increases top by 1
void push(Stack stack, int item)
{
    if (isFull(stack))
        return;
         
    stack.array[++stack.top] = item;
}
 
// Function to remove an item from stack.It
// decreases top by 1
int pop(Stack stack)
{
    if (isEmpty(stack))
        return Integer.MIN_VALUE;
         
    return stack.array[stack.top--];
}
 
// Function to implement legal movement between
// two poles
void moveDisksBetweenTwoPoles(Stack src, Stack dest,
                              char s, char d)
{
    int pole1TopDisk = pop(src);
    int pole2TopDisk = pop(dest);
 
    // When pole 1 is empty
    if (pole1TopDisk == Integer.MIN_VALUE)
    {
        push(src, pole2TopDisk);
        moveDisk(d, s, pole2TopDisk);
    }
     
    // When pole2 pole is empty
    else if (pole2TopDisk == Integer.MIN_VALUE)
    {
        push(dest, pole1TopDisk);
        moveDisk(s, d, pole1TopDisk);
    }
     
    // When top disk of pole1 > top disk of pole2
    else if (pole1TopDisk > pole2TopDisk)
    {
        push(src, pole1TopDisk);
        push(src, pole2TopDisk);
        moveDisk(d, s, pole2TopDisk);
    }
    // When top disk of pole1 < top disk of pole2
    else
    {
        push(dest, pole2TopDisk);
        push(dest, pole1TopDisk);
        moveDisk(s, d, pole1TopDisk);
    }
}
 
// Function to show the movement of disks
void moveDisk(char fromPeg, char toPeg, int disk)
{
    System.out.println("Move the disk " + disk +
                            " from " + fromPeg +
                              " to " + toPeg);
}
 
// Function to implement TOH puzzle
void tohIterative(int num_of_disks, Stack
                  src, Stack aux, Stack dest)
{
    int i, total_num_of_moves;
    char s = 'S', d = 'D', a = 'A';
  
    // If number of disks is even, then
    // interchange destination pole and
    // auxiliary pole
    if (num_of_disks % 2 == 0)
    {
        char temp = d;
        d = a;
        a  = temp;
    }
    total_num_of_moves = (int)(Math.pow(
                         2, num_of_disks) - 1);
  
    // Larger disks will be pushed first
    for(i = num_of_disks; i >= 1; i--)
        push(src, i);
  
    for(i = 1; i <= total_num_of_moves; i++)
    {
        if (i % 3 == 1)
          moveDisksBetweenTwoPoles(src, dest, s, d);
  
        else if (i % 3 == 2)
          moveDisksBetweenTwoPoles(src, aux, s, a);
  
        else if (i % 3 == 0)
          moveDisksBetweenTwoPoles(aux, dest, a, d);
    }
}
 
// Driver code
public static void main(String[] args)
{
     
    // Input: number of disks
    int num_of_disks = 3;
     
    TOH ob = new TOH();
    Stack src, dest, aux;
     
    // Create three stacks of size 'num_of_disks'
    // to hold the disks
    src = ob.createStack(num_of_disks);
    dest = ob.createStack(num_of_disks);
    aux = ob.createStack(num_of_disks);
     
    ob.tohIterative(num_of_disks, src, aux, dest);
}
}
 
// This code is contibuted by Sumit Ghosh

C#

// C# program for iterative
// Tower of Hanoi
using System;
 
class GFG
{
    // A structure to represent a stack
    public class Stack
    {
        public int capacity;
        public int top;
        public int []array;
    }
     
    // function to create a stack of given capacity.
    Stack createStack(int capacity)
    {
        Stack stack = new Stack();
        stack.capacity = capacity;
        stack.top = -1;
        stack.array = new int[capacity];
        return stack;
    }
     
    // Stack is full when top is equal to the last index
    Boolean isFull(Stack stack)
    {
        return (stack.top == stack.capacity - 1);
    }
     
    // Stack is empty when top is equal to -1
    Boolean isEmpty(Stack stack)
    {
        return (stack.top == -1);
    }
     
    // Function to add an item to stack.
    // It increases top by 1
    void push(Stack stack,int item)
    {
        if(isFull(stack))
            return;
        stack.array[++stack.top] = item;
    }
     
    // Function to remove an item from stack. 
    // It decreases top by 1
    int pop(Stack stack)
    {
        if(isEmpty(stack))
            return int.MinValue;
        return stack.array[stack.top--];
    }
     
    // Function to implement legal 
    // movement between two poles
    void moveDisksBetweenTwoPoles(Stack src, Stack dest,
                                            char s, char d)
    {
        int pole1TopDisk = pop(src);
        int pole2TopDisk = pop(dest);
 
        // When pole 1 is empty
        if (pole1TopDisk == int.MinValue)
        {
            push(src, pole2TopDisk);
            moveDisk(d, s, pole2TopDisk);
        }
         
        // When pole2 pole is empty
        else if (pole2TopDisk == int.MinValue)
        {
            push(dest, pole1TopDisk);
            moveDisk(s, d, pole1TopDisk);
        }
         
        // When top disk of pole1 > top disk of pole2
        else if (pole1TopDisk > pole2TopDisk)
        {
            push(src, pole1TopDisk);
            push(src, pole2TopDisk);
            moveDisk(d, s, pole2TopDisk);
        }
         
        // When top disk of pole1 < top disk of pole2
        else
        {
            push(dest, pole2TopDisk);
            push(dest, pole1TopDisk);
            moveDisk(s, d, pole1TopDisk);
        }
    }
     
    // Function to show the movement of disks
    void moveDisk(char fromPeg, char toPeg, int disk)
    {
        Console.WriteLine("Move the disk "+disk +
                        " from "+fromPeg+" to "+toPeg);
    }
     
    // Function to implement TOH puzzle
    void tohIterative(int num_of_disks, Stack
                src, Stack aux, Stack dest)
    {
        int i, total_num_of_moves;
        char s = 'S', d = 'D', a = 'A';
     
        // If number of disks is even, then interchange
        // destination pole and auxiliary pole
        if (num_of_disks % 2 == 0)
        {
            char temp = d;
            d = a;
            a = temp;
        }
        total_num_of_moves = (int) (Math.Pow(2, num_of_disks) - 1);
     
        // Larger disks will be pushed first
        for (i = num_of_disks; i >= 1; i--)
            push(src, i);
     
        for (i = 1; i <= total_num_of_moves; i++)
        {
            if (i % 3 == 1)
            moveDisksBetweenTwoPoles(src, dest, s, d);
     
            else if (i % 3 == 2)
            moveDisksBetweenTwoPoles(src, aux, s, a);
     
            else if (i % 3 == 0)
            moveDisksBetweenTwoPoles(aux, dest, a, d);
        }
    }
     
    // Driver code
    public static void Main(String []args)
    {
         
        // Input: number of disks
        int num_of_disks = 3;
         
        GFG ob = new GFG();
        Stack src, dest, aux;
         
        // Create three stacks of size 'num_of_disks'
        // to hold the disks
        src = ob.createStack(num_of_disks);
        dest = ob.createStack(num_of_disks);
        aux = ob.createStack(num_of_disks);
         
        ob.tohIterative(num_of_disks, src, aux, dest);
    }
}
 
// This code is Contributed by Arnab Kundu

Output:

Move the disk 1 from 'S' to 'D'
Move the disk 2 from 'S' to 'A'
Move the disk 1 from 'D' to 'A'
Move the disk 3 from 'S' to 'D'
Move the disk 1 from 'A' to 'S'
Move the disk 2 from 'A' to 'D'
Move the disk 1 from 'S' to 'D'

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References:
http://en.wikipedia.org/wiki/Tower_of_Hanoi#Iterative_solution
This article is contributed by Anand Barnwal. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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