recursive version of catenate

IntList.java

public class IntList {
  public int first;
  public IntList rest;
  
  public IntList(int first0, IntList rest0) {
    first = first0;
    rest = rest0;
  }
  
  /**
   * Returns a list consisting of the elements of A followed by the
   * * elements of B.  May NOT modify items of A.  Use 'new'.
   */
  public static IntList catenate(IntList A, IntList B) {
    IntList M = null;
    IntList p = null;
    if (A != null) {
      M = new IntList(A.first, null);
      p = M;
      A = A.rest;
      while (A != null) {
        p.rest = new IntList(A.first, null);
        p = p.rest;
        A = A.rest;
      } /* now p points to the last element of A */
    } else if (B != null) { /* A == null */
    // if (A == null && B != null) {  /* this line is bug. A is changed */
      M = new IntList(B.first, null);
      p = M;
      B = B.rest;
    }
    while (B != null) { /* can't put this block inside the above block */
      p.rest = new IntList(B.first, null);
      p = p.rest;
      B = B.rest;
    }
    return M;
  }

  /**
   * catenate - recursive
   */
  public static IntList catenate_re(IntList A, IntList B) {
    if (A == null) {
      /* Enter B mode! later A should be null always */
      if (B == null) {
        return null;
      }
      return new IntList(B.first, catenate_re(null, B.rest));
    } else {
      /* Enter A mode! later B stays the same */
      return new IntList(A.first, catenate_re(A.rest, B));
    }
  }
  
  public static main(String[] args) {
    IntList L1 = IntList.of(1, 2, 3, 4, 5);
    IntList L2 = IntList.of(6, 7);
    System.out.println(catenate_re(L1, L2));
  }
  
  
  // ----------------------------------------------------------------------
  
  
  public static IntList of(Integer... args) {
    IntList result, p;

    if (args.length > 0) {
      result = new IntList(args[0], null);
    } else {
      return null;
    }

    int k;
    for (k = 1, p = result; k < args.length; k += 1, p = p.rest) {
      p.rest = new IntList(args[k], null);
    }
    return result;
  }
  
  @Override
  /** Outputs the IntList as a String. You are not expected to read
   * or understand this method. */
  public String toString() {
    Formatter out = new Formatter();
    String sep;
    sep = "(";
    int cycleLocation = detectCycles(this);
    int cnt = 0;

    for (IntList p = this; p != null; p = p.rest) {
      out.format("%s%d", sep, p.first);
      sep = ", ";

      cnt++;
      if ((cnt > cycleLocation) && (cycleLocation > 0)) {
        out.format("... (cycle exists) ...");
        break;
      }
    }
    out.format(")");
    return out.toString();
  }
}