Permutation of an array, with repetition, in Java

Question

There are some similar questions on the site that have been of some help, but I can't quite nail down this problem, so I hope this is not repetitive.

This is a homework assignment where you have a set array of characters [A, B, C], and must use recursion to get all permutations (with repetition). The code I have sort of does this:

char[] c = {'A', 'B' , 'C'};

public void printAll(char[] c, int n, int k) {
    if (k == n) {
      System.out.print(c);
      return;
    }
    else {   
      for (int j = 0; j<n; j++) {
        for (int m = 0; m<n; m++) {
           System.out.print(c[k]); 
           System.out.print(c[j]); 
           System.out.print(c[m] + "\r\n");
        }
      }
    }        
    printAll(c, n, k+1);    
}

However, the parameter n should define the length of the output, so while this function prints out all permutations of length 3, it cannot do them of length 2. I have tried everything I can think of, and have pored over Google search results, and I am aggravated with myself for not being able to solve what seems to be a rather simple problem.

Answer 1

If I understand correctly, you are given a set of characters c and the desired length n.

Technically, there's no such thing as a permutation with repetition. I assume you want all strings of length n with letters from c.

You can do it this way:

to generate all strings of length N with letters from C
 -generate all strings of length N with letters from C
     that start with the empty string.

to generate all strings of length N with letters from C
   that start with a string S
 -if the length of S is N
  -print S
 -else for each c in C
  -generate all strings of length N with letters from C that start with S+c

In code:

printAll(char[] c, int n, String start){
  if(start.length >= n){
    System.out.println(start)
  }else{
    for(char x in c){ // not a valid syntax in Java
      printAll(c, n, start+x);
    }
  }
}