Merge Sort Implementation Issues

Question

I'm attempting to implement a merge sort in Java for my Algorithms class. I've tried several approaches, including assigning the last array indices in both arrays as "infinity" using Java's Double.POSITIVE_INFINITY value typecasted as an integer.

import java.util.Arrays;

public static int[] mergeSort(int[] arr, int p, int r){
  int[] sortedArray = new int[1];
  if(p<r){
    int q = (int)Math.floor((p+r)/2);

    mergeSort(arr,p,q);
    mergeSort(arr,q+1,r);
    sortedArray = merge(arr,p,q,r);
  }

  return sortedArray;
}

public static int[] merge(int[] arr, int p, int q, int r){

  //I've tested here with an array of size two, and left returns an array
  //length of zero. I'm not sure if it might have to do with how the base case
  //of size 1 arrays is handled.
  int[] left = Arrays.copyOfRange(arr,p,q);
  int[] right = Arrays.copyOfRange(arr,q+1,r);

  int i = 0, j = 0;
  int k;

  //merge the two arrays; check to see if the i or j indexes are out of range
  //before attempting to merge
  for(k = p; k <= r-1 && i < left.length && j < right.length; k++){
    if(left[i] <= right[j]){
      arr[k] = left[i];
      i++;
    }else{
      arr[k] = right[j];
      j++;
    }
  }

  //I was handling these assignments in the loop above, but my index variables were going 
  //out of range when Arrays.copyOfRange() returned arrays of length 0.
  if(i >= left.length && j < right.length){
    while(j < right.length){
      arr[k] = right[j];
      j++;
    }
  }else if(j >= right.length && i < left.length){
    while(i < left.length){
      arr[k] = left[i];
      i++;
    }
  }

  return arr;
}

I'm thinking my issue is just a couple of index values that are off by a bit, but I can't figure out where they would be for the life of me.

Answer 1

In your call Arrays.copyOfRange(arr,p,q) , the range extends from p (inclusive) to q (exclusive).

If p and q are equal, the result will be a zero-length array.

In your own method signatures, consider and document whether the endpoint r is inclusive or exclusive.

Also, consider what happens when your test if(p<r) fails in mergesort() -- you return an new 1-element array without changing its members from the default.