الگوریتم مرتب سازی ادغامی (Merge Sort)

public class  		Merge {
		    public static void sort(Comparable[] a) {
		        sort(a, 0, a.length);
		    }
		    // Sort a[lo, hi).
		    public static void sort(Comparable[] a, int lo, int hi) {
		        int N = hi - lo;        // number of elements to sort
		
		        // 0- or 1-element file, so we're done
		        if (N <= 1) return;
		
		        // recursively sort left and right halves
		        int mid = lo + N/2;
		        sort(a, lo, mid);
		        sort(a, mid, hi);
		
		        // merge two sorted subarrays
		        Comparable[] aux = new Comparable[N];
		        int i = lo, j = mid;
		        for (int k = 0; k < N; k++) {
		            if      (i == mid)  aux[k] = a[j++];
		            else if (j == hi)   aux[k] = a[i++];
		            else if (a[j].compareTo(a[i]) < 0) aux[k] = a[j++];
		            else                               aux[k] = a[i++];
		        }
		
		        // copy back
		        for (int k = 0; k < N; k++) {
		            a[lo + k] = aux[k];
		        }
		    }
		
		    		/***********************************************************************
		    *  Check if array is sorted - useful for debugging
		     		***********************************************************************/
		    private static boolean isSorted(Comparable[] a) {
		        for (int i = 1; i < a.length; i++)
		            if (a[i].compareTo(a[i-1]) < 0) return false;
		        return true;
		    }
		
		// test client
		    public static void main(String[] args) {
		        int N = 10;
		
		        // generate N random real numbers between 0 and 1
		        long start = System.currentTimeMillis();
		        Double[] a = new Double[N];
		        for (int i = 0; i < N; i++)
		            a[i] = Math.random();
		
		        System.out.print("primaray array is: \n");
		        for(int k=0; k<N; k++){
		            System.out.print(a[k]);
		            if (k<9)
		               System.out.print(" , ");
		        }
		        System.out.print("\n");
		
		        long stop = System.currentTimeMillis();
		        double elapsed = (stop - start) / 1000.0;
		        System.out.println("Generating input:  " + elapsed + "  		seconds");
		
		        // sort them
		        start = System.currentTimeMillis();
		        sort(a);
		        stop = System.currentTimeMillis();
		        elapsed = (stop - start) / 1000.0;
		        System.out.print("\nsorted array is: \n ");
		        for(int k=0; k<N; k++){
		            System.out.print(a[k]);
		            if (k<9)
		               System.out.print(" , ");
		        }
		        System.out.print("\n");
		        System.out.print("Start sort time (in mili second) is: " + start  		+"\n");
		        System.out.print("end sort time (in mili second) is: " + stop  		+"\n");
		
		        System.out.println("Mergesort: " + elapsed + " seconds");
		        System.out.println(isSorted(a));
		
		        
		    }
		}