Java中的排序合并代码需要一些澄清

您能否在下面解释几行（我在每行需要解释的前面加上？）。提前致谢 ！我知道：对具有n个元素的输入序列S进行合并排序包括三个步骤：划分：将S划分为两个序列S1和S2，每个序列分别约n / 2个元素递归：递归地对S1和S2进行排序征服：合并S1和S2成为唯一的排序序列

但是，当我阅读代码时，我迷失了方向，请您指导我。

public class MergeSort {
        public static int[] mergeSort(int [] list) {
            if (list.length <= 1) {
                return list;
            }

            // Split the array in half
            int[] first = new int[list.length / 2];  // ok
            int[] second = new int[list.length - first.length]; // ?
            System.arraycopy(list, 0, first, 0, first.length);  // ?
            System.arraycopy(list, first.length, second, 0, second.length); // not sure ?

            // Sort each half
            mergeSort(first); // ok
            mergeSort(second); // ok

            // Merge the halves together, overwriting the original array
            merge(first, second, list); // ok
            return list; // ok
        }

        private static void merge(int[] first, int[] second, int [] result) { // explain in general ?
            // Merge both halves into the result array
            // Next element to consider in the first array
            int iFirst = 0;
            // Next element to consider in the second array
            int iSecond = 0;

            // Next open position in the result
            int j = 0;
            // As long as neither iFirst nor iSecond is past the end, move the
            // smaller element into the result.
            while (iFirst < first.length && iSecond < second.length) { // ??!!
                if (first[iFirst] < second[iSecond]) {
                    result[j] = first[iFirst];
                    iFirst++;
                    } else {
                    result[j] = second[iSecond];
                    iSecond++;
                }
                j++;
            }
            // copy what's left
            System.arraycopy(first, iFirst, result, j, first.length - iFirst);
            System.arraycopy(second, iSecond, result, j, second.length - iSecond);
        }

        public static void main(String args[]) throws Exception
        {
            String list="";
            int i=0,n=0;

            MergeSort s= new MergeSort();
            ArrayList<Integer> arrlist=new ArrayList<Integer>();
            System.out.println(" ");
            System.out.println(" ");
            System.out.println("Please enter the list of elements,one element per line");
            System.out.println(" write 'STOP' when list is completed ");
            BufferedReader bf=new BufferedReader(new InputStreamReader(System.in));
            while(!(list=bf.readLine()).equalsIgnoreCase("stop")){
                int intelement=Integer.parseInt(list);
                arrlist.add(intelement);

            }

            int elementlist[]  = new int[arrlist.size()];
            Iterator<Integer> iter = arrlist.iterator();
            for (int j=0;iter.hasNext();j++) {
                elementlist[j] = iter.next();
            }

            elementlist=mergeSort(elementlist); // ?
            System.out.println(" ");
            System.out.println(" ");
            System.out.println(" ");
            System.out.println("Values after Merge Sort : ");
            for (int j=0;j<elementlist.length;j++) {
                System.out.println(elementlist[j]+" ");
            }
        }
    }