Merge works as follows.

• The values in the top two arrays are merged into the result array at the bottom.
• In each step of merging, the smallest element in the unmerged parts of the top two arrays is picked up and is added to the result array.
• The merging proceeds until the elements in the top two arrays are exhausted.
• We use idx1 to designate the next element to be merged from the first array, and idx2 to designate the next element to be merged from the second array.