Java 归并排序

import java.util.Random;

/**
 * 
 * @author  liebaorun liu
 */
public class MergeSortUtil {

	
	public static void mergeSort(int[] arr){
		int[] arrCopy = new int[arr.length];
		mergeSort(arr,arrCopy,0,arr.length-1);
	}
	
	
	private static void mergeSort(int[] arr, int[] arrCopy, int start, int end){
		
		if(start < end){

			int middle = (start+end)/2;
			mergeSort(arr,arrCopy,start,middle);
			mergeSort(arr,arrCopy,middle+1,end);

			//合并已排序的两个数组
			{
				int i=start, j=middle+1, pointer=start;
				for( ; i<=middle && j<=end ; ){
					if(arr[i] < arr[j]){
						arrCopy[pointer++] = arr[i++];
					}else{
						arrCopy[pointer++] = arr[j++];
					}
				}
				if(i > middle){
					System.arraycopy(arr, j, arrCopy, pointer, end-j+1);
				} else {
					System.arraycopy(arr, i, arrCopy, pointer, middle-i+1);
				}
			}
			
			//反Copy已排序的临时数组到源数组
			System.arraycopy(arrCopy, start, arr, start, end-start+1);
		}
	}
	
	public static void main(String[] args){
		int size = 20;
		int[] arr = new int[size];
		
		Random random = new Random();
		for(int i=0; i< arr.length; i++){
			arr[i] = random.nextInt(20);
			System.out.print(arr[i]+ " ");
		}

		mergeSort(arr);
		
		System.out.println();
		
		for(int i=0; i< arr.length; i++){
			//arr[i] = random.nextInt(20);
			System.out.print(arr[i]+ " ");
		}
		
	}
	
}