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Towers of Hanoi

In this exercise, you will implement the Towers of Hanoi puzzle with a recursive algorithm. The aim of this game is to transfer all the disks from one of the three rods to another, following these rules:

  • You can only move one disk at a time.
  • You can only take the upper disk from one of the stacks and place it on top of another stack.
  • You cannot put a larger disk on top of a smaller one.

Picture of the game Tower of Hanoi

The algorithm shown is an implementation of this game with four disks and three rods called 'A', 'B' and 'C'. The code contains two mistakes. In fact, if you execute it, it crashes the console because it exceeds the maximum recursion depth. Can you find the bugs and fix them?

This exercise is part of the course

Data Structures and Algorithms in Python

View Course

Exercise instructions

  • Correct the base case.
  • Correct the calls to the hanoi() function.

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

def hanoi(num_disks, from_rod, to_rod, aux_rod):
  # Correct the base case
  if num_disks >= 0:
    # Correct the calls to the hanoi function
    hanoi(num_disks, from_rod, aux_rod, to_rod)
    print("Moving disk", num_disks, "from rod", from_rod,"to rod",to_rod)
    hanoi(num_disks, aux_rod, to_rod, from_rod)   

num_disks = 4
source_rod = 'A'
auxiliar_rod = 'B'
target_rod = 'C'

hanoi(num_disks, source_rod, target_rod, auxiliar_rod)
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