Permutations excluding repeated characters

Question

I\'m working on a Free Code Camp problem - http://www.freecodecamp.com/challenges/bonfire-no-repeats-please

The problem description is as follows -

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Answer 1

Well I won't have any mathematical solution for you here.

I guess you know backtracking as I percieved from your answer.So you can use Backtracking to generate all permutations and skipping a particular permutation whenever a repeat is encountered. This method is called Backtracking and Pruning.

Let n be the the length of the solution string, say(a1,a2,....an). So during backtracking when only partial solution was formed, say (a1,a2,....ak) compare the values at ak and a(k-1). Obviously you need to maintaion a reference to a previous letter(here a(k-1))

If both are same then break out from the partial solution, without reaching to the end and start creating another permutation from a1.