Question

Our teacher gave us the pseudocode for Merge Sort as below

I want to implement it in Java. My code is below:

public class MergeSorter {
/**
 * @param anArray
*/
    public MergeSorter(int[] anArray,int low, int high)
    {
        a = anArray;
        p = low;
        r = high;
    }
    public void sort()
    {
        if(p < r)
        {
           q = (p + r)/2;
           MergeSorter pqSorter = new MergeSorter(a, p, q);
           MergeSorter qrSorter = new MergeSorter(a, q + 1, r);
           pqSorter.sort();
           qrSorter.sort();
           merge(p, q, r);
        }
   }
private void merge(int low, int mid, int high)
{
   p = low;
   q = mid;
   r = high;
   int i;
   int j;
   int n1 = (q - p) + 1;
   int n2 = (r - q);
   int[] L = new int[n1+1];
   for (i = 0; i < n1; i++)
   {
       L[i] = a[(p + i)];
   }
   int[] R = new int[n2+1];
   for (j = 0; j < n2; j++)
       {
       R[j] = a[q + j];
       }
       L[n1] = Integer.MAX_VALUE;
       R[n2] = Integer.MAX_VALUE;
       i = 0;
       j = 0;
       for (int k = p; k < r; k++)
       {
           count = count + 1;
           if(L[i] <= R[j])
           {
               a[k] = L[i];
               i = i + 1;
           }
          else
           {
               a[k] = R[j];
               j = j + 1;
          }
       } 
    }
    private int[] a;
    private int p; 
    private int r;
    private int q;
    public int count = 0;
}

But this code is not working.I want to know where is the problem. Sorry for the wrong direction picture.
Updated: Here is my other code.It now dose something but not sorted right.
This is my test code below

    public static void main(String[] args) throws FileNotFoundException, IOException
    {
      int[] a = {1,4,6,2,10,7};
      MergeSorter sorter = new MergeSorter(a,0,a.length);
      sorter.sort();
      System.out.println(Arrays.toString(a));
     }        

It output [1,2,4,4,2,7] as a result.

Answer 1

You have a couple of off-by-one errors.

MergeSorter sorter = new MergeSorter(a,0,a.length);

The last index of an array is a.length - 1.

for (j = 0; j < n2; j++)
{
    R[j] = a[q + j];
}

The second half to be merged runs from index q+1 to r.

for (int k = p; k < r; k++)

The last index to be filled is r, not r-1, so the condition should be k <= r.

Further, the setting of the instance variables

p = low;
q = mid;
r = high;

in merge() is fishy. It doesn’t hurt here because they are set to the values they already have, but in principle it’s wrong.