Why use a merge sort?

• Merge sort is especially useful when one wishes to combine two lists into a sorted list.

How does a merge sort work?

• A merge sort works by breaking a list into two different lists, sorting each list, and finally combining them together.  A merge sort can also continue to break a list into new lists until the new lists are of length two.

Want some code?

//merge sort
class MergeSortAlgorithm extends SortAlgorithm
{
    void sort(int a[], int lo0, int hi0) throws Exception
    {
    int lo = lo0;
    int hi = hi0;
    pause(lo, hi);
    if (lo >= hi)
    {
        return;
    }
    int mid = (lo + hi) / 2;

        /*
         *  Partition the list into two lists and sort them recursively
         */
        sort(a, lo, mid);
        sort(a, mid + 1, hi);

        /*
         *  Merge the two sorted lists
         */
        int end_lo = mid;
        int start_hi = mid + 1;
        while ((lo <= end_lo) && (start_hi <= hi))
        {
            pause(lo);
            if (stopRequested)
            {
                return;
            }
            if (a[lo] < a[start_hi])
            {
                lo++;
            }
            else
            {
                /*  
                 *  a[lo] >= a[start_hi]
                 *  The next element comes from the second list,
                 *  move the a[start_hi] element into the next
                 *  position and shuffle all the other elements up.
                 */
                int T = a[start_hi];
                for (int k = start_hi - 1; k >= lo; k--)
                {
                    a[k+1] = a[k];
                    pause(lo);
                }
                a[lo] = T;
                lo++;
                end_lo++;
                start_hi++;
            }
        }
    }

    void sort(int a[])  throws Exception
    {
        //recursion
        sort(a, 0, a.length-1);
    }
}

Notes by Alex Loddengaard