FAQ: Merge Sort: Java - Merging

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This community-built FAQ covers the “Merging” 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

FAQs on the exercise Merging

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2

The exercise uses a merge function that checks whether the current element in the left array is smaller than the current element in the right array, and puts the smallest of them into the merged array.
It checks the integer at each index.

But I thought it was only necessary to check the first element in each array (the stuff at index 0) for the merging in a “Merge Sort”.
I came up with a merge method that would do that:

    public int[] mergeByLowestFirstNum(int[] left, int[] right) {
      if ((right == null) || (right.length == 0)) {
        return left;
      }
      else if ((left == null) || (left.length == 0)) {
        return right;
      }
      int[] merged = new int[left.length + right.length];
      // switch left and right if first element of right is smallest
      if (right[0] < left[0]) { 
        int[] temp = left;
        left = right;
        right = temp;
      }
      
      int curIndex = 0;
      // put stuff from left into merged
      for (int leftPos = 0; leftPos < left.length; leftPos++) {
        merged[curIndex] = left[leftPos];
        curIndex++;
      }
      // put stuff from right into merged
      for (int rightPos = 0; rightPos < right.length; rightPos++) {
        merged[curIndex] = right[rightPos];
        curIndex++;
      }

      return merged;
    }

And I guess those for-loops could be replaced with

      System.arraycopy(left, 0, merged, 0, left.length);  // copy left into merged
      System.arraycopy(right, 0, merged, left.length, right.length);  // copy right into merged
4

I think that fails in some very specific cases, like for [5,4,3,2,1,0,6,8]