Python - RecursionError - maximum recursion depth exceeded

Question

I'm a student working on this homework problem. The code tries to find a solution to any given circular board game, where a list is given as an argument. The values in the list tell the program how far to move clockwise/counterclockwise (last element is always 0).

The program is meant to return True if there is a solution to that particular puzzle and False otherwise. Except in the case of having 0 as the first and only element, my code returns a RecursionError even for short lists. What's wrong with my code?

def solve_puzzle(L): # uses helper for memoization
    visited = set() # visited indices
    return _solve_puzzle(L, visited, 0)

def _solve_puzzle(L, visited, index): # always start at first element
    if L[index] not in visited:
        visited.add(index)
        if L[index] == 0: 
            return True
        else:
            # loop around to the end or start in python
            if L[index] + index >= len(L):
                new_index_add = index + L[index] - len(L)
            else:
                new_index_add = index + L[index]
            
            if index - L[index] < 0:
                new_index_sub = len(L) - L[index] - index
            else:
                new_index_sub = index - L[index]
                
            # need to figure out the indexing - how do we go both backward and forward
            return _solve_puzzle(L, visited, new_index_add) or _solve_puzzle(L, visited, new_index_sub) # True if it's solveable in either direction
        
    return False

Dharman · Accepted Answer · 2022-02-26 17:18:31Z

0

You should probably add visited = visited.copy() at the top of the function. Otherwise, when visited is modified, it gets modified for every call, since sets are mutable.

I can see two other tiny mistakes, but I think you should figure them out on your own. I don't want to give you the whole answer since it's a homework task. Fix them and it should work. Sorry.

edited Feb 26, 2022 at 17:18

Dharman♦

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answered Feb 26, 2022 at 17:13

Charlie

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