/**
 * Here I show how you can generate all the combinations of a sequence
 * of size r which are repeated at most k times.
 * 
 * @author William Fiset, william.alexandre.fiset@gmail.com
 **/

public class CombinationsWithRepitition {
  
  /** 
   * Computes all combinations of elements of 'r' elements which 
   * can be repeated at most 'k' times each.
   * @param sequence   - The sequence containing all the elements we wish to take combinations from
   * @param usedCount  - Tracks how many of each element we currently have selected
   * @param at         - The current position we're at in the sequence
   * @param r          - The number of elements we're choosing
   * @param k          - The meximum number of times each element is allowed to be picked
   **/
  private static void combinationsWithRepetition(int[] sequence, int[] usedCount, int at, int r, int k) {

    final int N = sequence.length;

    // We reached the end
    if (at == N) {

      // We selected 'r' elements in total
      if (r == 0) {

        // Print combination
        System.out.print("{ ");
        for (int i = 0; i < N; i++)
          for (int j = 0; j < usedCount[i]; j++)
            System.out.print(sequence[i] + " ");
        System.out.println("}");

      }

    } else {

      // For this particular time at position 'at' try including it each of [0, k] times
      for (int itemCount = 0; itemCount <= k; itemCount++) {

        // Try including this element itemCount number of times (this is possibly more than once)
        usedCount[at] = itemCount;

        combinationsWithRepetition( sequence, usedCount, at + 1, r - itemCount, k);

      }

    }
  }

  // Given a sequence this method prints all the combinations of size
  // 'r' in a given sequence which was each element repeated at most 'k' times
  public static void printCombinationsWithRepetition(int[] sequence, int r, int k) {

    if (sequence == null) return;
    final int n = sequence.length;
    if (r > n) throw new IllegalArgumentException("r must be <= n");
    if (k > r) throw new IllegalArgumentException("k must be <= k");

    int[] usedCount = new int[sequence.length];
    combinationsWithRepetition( sequence, usedCount, 0, r, k );

  }

  public static void main(String[] args) {
    
    // Prints all combinations of size 3 where 
    // each element is repeated at most twice
    int[] seq = {1,2,3,4};
    printCombinationsWithRepetition(seq, 3, 2);
    // prints:
    // { 3 4 4 }
    // { 3 3 4 }
    // { 2 4 4 }
    // { 2 3 4 }
    // { 2 3 3 }
    // { 2 2 4 }
    // { 2 2 3 }
    // { 1 4 4 }
    // { 1 3 4 }
    // { 1 3 3 }
    // { 1 2 4 }
    // { 1 2 3 }
    // { 1 2 2 }
    // { 1 1 4 }
    // { 1 1 3 }
    // { 1 1 2 }

  }

}

@@ -698,17 +698,17 @@
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Update CombinationsWithRepitition.java