FAQ: Merge Sort: Java - Review

Frequently Asked Questions Java FAQ
1

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This community-built FAQ covers the “Review” exercise from the lesson “Merge Sort: Java”.

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This exercise can be found in the following Codecademy content:

Pass the Technical Interview with Java

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2

So I started lookning at the addtional challenge to combine the two lines inside the if else statements into one, and thought what a great moment to practice what I’ve learnt about ternary operators. I went a little further and put everything in one line, such that:

while (leftPos < left.length && rightPos < right.length) {
          merged[curIndex] = left[leftPos] < right[rightPos] ? left[leftPos++] : right[rightPos++];
          
          curIndex++;
        }

The best part is it worked!! Proud noob moment. :slight_smile:

3

I tried to rewrite the code to work for an ArrayList instead of an array.
I created copyOfRange and arraycopy​ functions for ArrayLists so that I wouldn’t need to change calling those.

code 
    public ArrayList<Integer> sort(ArrayList<Integer> arr) {
      int length = arr.size();
      if (length <= 1) {
        return arr;
      }
      int mid = Math.floorDiv(length, 2);
      ArrayList<Integer> leftArray = copyOfRange(arr, 0, mid);
      ArrayList<Integer> rightArray = copyOfRange(arr, mid, length);
      return merge(sort(leftArray), sort(rightArray));
    }
    
    public static ArrayList<Integer> merge(ArrayList<Integer> left, ArrayList<Integer> right) {
        int length = left.size() + right.size();
        ArrayList<Integer> merged = new ArrayList<Integer>(length);
        int leftPos = 0;
        int rightPos = 0;
        int curIndex = 0;

        while (leftPos < left.size() && rightPos < right.size()) {
            if (left.get(leftPos) < right.get(rightPos)) {
                merged.add(left.get(leftPos));
                leftPos++;
            } else {
                merged.add(right.get(rightPos));
                rightPos++;
            }
            curIndex++; // same as merged.size()
        }

        arraycopy(left, leftPos, merged, curIndex, left.size() - leftPos);
        arraycopy(right, rightPos, merged, curIndex, right.size() - rightPos);
        return merged;
    }
    
    public static ArrayList<Integer> copyOfRange(ArrayList<Integer> arr, int fromIndex, int toIndex) {
      ArrayList<Integer> copy = new ArrayList<Integer>(toIndex - fromIndex);
      copy.addAll(arr.subList(fromIndex, toIndex));
      return copy;
    }
    
    public static ArrayList<Integer> arraycopy​(ArrayList<Integer> src, int srcPos, ArrayList<Integer> dest, int destPos, int length) {
      dest.addAll(destPos, src.subList(srcPos, srcPos + length));
      return dest;
    }
other version of merge method

A version of merge that concatenates the arraylists more directly.

    public static ArrayList<Integer> merge(ArrayList<Integer> left, ArrayList<Integer> right) {
      int length;
      if (left == null || left.size() == 0) {
        return right;
      }
      else if (right == null || right.size() == 0) {
        return left;
      }
      else {
        length = left.size() + right.size();
      }
      if (left.get(0) <= right.get(0)) {
        left.ensureCapacity(length);
        left.addAll(right);
        return left;
      }
      else {
        right.ensureCapacity(length);
        right.addAll(left);
        return right;
      }
    }
4

I guess that second version of the merge method does not work correctly.