8

I read this simple and elegant python solution for finding all permutations of a given string. It is recursive. Based on that I tried to implement an iterative solution in python.

Below is my code. But it works only for 3 character strings :( Stuck trying to see how the recursion base case condition and recursion condition translates into iterative(non-recursive) Any pointers would help to get a iterative solution working.(Either based on this algorithm or any other)

def  permutations_iter(word):
while True:
    perms = []
    result = []

    char = word[0]
    new_word = word[1:]

    if len(new_word)==2: 
        perms = [new_word,''.join(reversed(new_word))]

    for perm in perms: 
        #insert the character into every possible location 
        for i in range(len(perm)+1): 
            result.append(perm[:i] + char + perm[i:]) 
    return result

    if len(new_word)==2:
        break;


   #example code to call this iterative function        
   print permutations_iter("LSE")
goldenmean
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1 Answers1

21

You can convert every recursion to an iteration using a stack. But in this case it's even simpler since the algorithm is very simple.

def perms(word):
    stack = list(word)
    results = [stack.pop()]
    while len(stack) != 0:
        c = stack.pop()
        new_results = []
        for w in results:
            for i in range(len(w)+1):
                new_results.append(w[:i] + c + w[i:])
        results = new_results
    return results

For a more general conversion of recursion to iteration with a stack read this

Community
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Viktor Kerkez
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