Generating all permutations of a certain length

Question

Suppose we have an alphabet \"abcdefghiklimnop\". How can I recursively generate permutations with repetition of this alphabet in groups of FIVE in an efficient way?

Answer 1

This is can be easily done using bit manipulation.

private void getPermutation(String str, int length)
        {
            if(str==null)
                return;
            Set StrList = new HashSet();
            StringBuilder strB= new StringBuilder();
            for(int i = 0;i < (1 << str.length()); ++i)
            {
                strB.setLength(0); //clear the StringBuilder
                if(getNumberOfOnes(i)==length){
                  for(int j = 0;j < str.length() ;++j){
                    if((i & (1 << j))>0){  // to check whether jth bit is set (is 1 or not)
                        strB.append(str.charAt(j));
                    }
                }
                StrList.add(strB.toString());
                }
            }
            System.out.println(Arrays.toString(StrList.toArray()));
        }

    private int getNumberOfOnes (int n) // to count how many numbers of 1 in binary representation of n
    {
        int count=0;
        while( n>0 )
        {
        n = n&(n-1);
           count++;
        }
        return count;
    }