Permutation of a String

Problem Statement :
    Print all the permutation of a given String.

Solution:

       Permutation in mathematics for n items take together at a time is  n!
We also know that  n! = n * (n-1) ! , which clearly gives an intuition that this problem can be solved using recursion.

Here the concept is to fix the 1st character , and generate permutation of the rest , then fix two characters and generate permutations of the rest , repeat this technique until all characters are processed. 
Supposing we have characters 'a' , ' b ' , ' c ' , ' d ' and we have to find permutation the given characters, so our approach will be as follows :

                   fix ' a ' followed by permutations of  ' b ' , ' c ' , ' d ' .
Similarly,   fix ' b ' followed by permutations of  ' a ' , ' c ' , ' d ' .
Similarly,   fix ' c ' followed by permutations of  ' a ' , ' b ' , ' d ' .
Similarly,   fix ' d ' followed by permutations of  ' a ' , ' b ' , ' c ' .

For e.g. permutation of "ABCD" are

                      

ABCD BACD CBAD DBCA ABDC BADC CBDA DBAC ACBD BCAD CABD DCBA ACDB BCDA CADB DCAB ADCB BDCA CDAB DACB ADBC BDAC CDBA DABC

In order to avoid repetition of  swapping of characters in case of duplicates , we are checking if the character is  in the range of indexes among which swapping is happening ,  if any character is found then we skip the current characters as that same character in that range will be processed at some later time during iteration , hence the match(int i , int j) subroutine helps to check the above condition.

Therefore in case of duplicate charaters in the string.
e.g. "AACD"

AACD ACDA CAAD DACA AADC ADCA CADA DAAC ACAD ADAC CDAA DCAA

Lets get on to the ground level. How are we gonna implement this ?? What would we require , loops , function calls or what other necessary things ?? before proceeding to write the code.
 We would implement the above logic by fixing every character at 1st position so we would obviously require a for loop running till the number of charaters in the String. Fixing a character at a position we repeat the process for the rest of characters by incrementing the start index , this process would be repeated until the start index has reached the length of the string , then we would print the string. This is just an overview to give an idea to proceed before you start coding.

Full Source Code:  LINK

Important sub-routines necessary for calculating permutations are shown below:

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private char[] str=null; // to hold string data /** * Subroutine to compute permutations * @param index * @param len */ private void permutate( int index, int len ) { if( index == len ) System.out.println((str)); for(int i = index; i <= len; i++ ) { if(!match(index, i)){ swap(index, i ); permutate(index + 1 , len); swap( index, i ); } } } /** * to check if any character matches between i and j * @return if any characted match found */ private boolean match(int i, int j) { if(i == j) return false; else for(;i<j ; i++) if(str[i]==str[j]) return true; return false; }

There is further scope of optimization of this code ,  so any suggestions or errata are most welcome.

Happy Coding  :)