Recursive method to print a descending then ascending integer sequence

From a programming assignment:


Write a method writeSequence that accepts an integer n as a
parameter and prints a symmetric sequence of n numbers with
descending integers ending in 1 followed by ascending integers
beginning with 1, as in the table below:

Call Output

writeSequence(1); 1
writeSequence(2); 1 1
writeSequence(3); 2 1 2
writeSequence(4); 2 1 1 2
writeSequence(5); 3 2 1 2 3
writeSequence(6); 3 2 1 1 2 3
writeSequence(7); 4 3 2 1 2 3 4
writeSequence(8); 4 3 2 1 1 2 3 4
writeSequence(9); 5 4 3 2 1 2 3 4 5
writeSequence(10); 5 4 3 2 1 1 2 3 4 5



Notice that for odd numbers the sequence has a single 1 in the middle
while for even values it has two 1s in the middle.


Your method should throw an IllegalArgumentException if passed a
value less than 1. A client using this method would have to call
println to complete the line of output.

I wrote code that is pretty ugly, but produces the correct output. I've put this up to see if anyone had a more efficient recursive algorithm and to put the example up. There are not many hits about more complex recursive methods in Java when searching.

`public static void writeSequence(int n) {

if( n

Assuming you can use a helper function, you could do this:

public static void writeSequence(int n) {
    if (n < 1) throw new IllegalArgumentException();
    writeSequenceHelper(n);
  }

  public static void writeSequenceHelper(int n) {
    if (n <= 0) return;
    int m = (n+1) / 2;
    System.out.print(m + " ");
    writeSequenceHelper(n - 2);
    System.out.print(m + " ");
  }

If you can't use a helper function like this to workaround the strange IllegalArgumentException requirement, you could do this (which is similar to your solution, but a little more succinct):

public static void writeSequence2(int n) {
    if (n < 1) throw new IllegalArgumentException();
    if (n == 1) {
      System.out.print("1");
    } else if (n == 2) {
      System.out.print("1 1");
    } else {
      int m = (n+1) / 2;
      System.out.print(m + " ");
      writeSequence2(n - 2);
      System.out.print(" " + m);
    }
  }

StackExchange Code Review Q#25964, answer score: 3