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String permutaion

I have found the following Java code. It counts all the permutations of a string. However, I can't understand what it does inside the for loop of the permutation method. More specifically I can't understand the purpose of the rem string and the recursive call.

Is there any thoughts on this?

Thanks.

void permutaion(String str){
    permutaion(str,"");
}

void permutaion(String str, String prefix){
    if(str.length() == 0){
        System.out.println(prefix);
    } else{
        for(int i=0; i < str.length(); i++){
            String rem = str.substring(0,i) + str.substring(i+1);
            permutaion(rem, prefix + str.charAt(i));
        }
    }
}

over 4 years ago · Santiago Trujillo

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You should run this code by hand on paper to see it's magic and understand it :) The recursive on permutation function is for permutation input string, and the for loop is for all possible case. The prefix is the permutation of the input string when a recursive finish.

over 4 years ago · Santiago Trujillo Report

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Accepted Answer · 2022-05-26T16:18:44.032Z

The basic idea is this: to get the permutations of a given N-character string you:

Take the first letter of the original string and append to it all the permutations of the remaining N - 1 characters;
Take the first two letters of the original string and append to them all the permutations of the remaining N - 2 characters;
Take the first three letters of the original string and append to them all the permutations of the remaining N - 3 characters ...

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String permutaion

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