# Towers of Hanoi

A Roc solution for the popular Towers of Hanoi problem.

## Code

interface Hanoi exposes [ hanoi, ] imports [] ## Solves the Tower of Hanoi problem using recursion. Returns a list of moves ## which represent the solution. hanoi : { numDisks : U32, # number of disks in the Tower of Hanoi problem from : Str, # identifier of the source rod to : Str, # identifier of the target rod using : Str, # identifier of the auxiliary rod moves : List (Str, Str), # list of moves accumulated so far } -> List (Str, Str) hanoi = \{ numDisks, from, to, using, moves } -> if numDisks == 1 then List.concat moves [(from, to)] else moves1 = hanoi { numDisks: (numDisks - 1), from, to: using, using: to, moves, } moves2 = List.concat moves1 [(from, to)] hanoi { numDisks: (numDisks - 1), from: using, to, using: from, moves: moves2, } start = { numDisks: 0, from: "A", to: "B", using: "C", moves: [] } ## Test Case 1: Tower of Hanoi with 1 disk expect hanoi { start & numDisks: 1 } == [("A", "B")] ## Test Case 2: Tower of Hanoi with 2 disks expect hanoi { start & numDisks: 2 } == [("A", "C"), ("A", "B"), ("C", "B")] ## Test Case 3: Tower of Hanoi with 3 disks expect hanoi { start & numDisks: 3 } == [("A", "B"), ("A", "C"), ("B", "C"), ("A", "B"), ("C", "A"), ("C", "B"), ("A", "B")]

## Output

Run this from the directory that has `Hanoi.roc`

in it:

$ roc test Hanoi.roc 0 failed and 3 passed in 662 ms.