归并排序 java实现

package sort;
//归并排序  O(logn*n)额外空间复杂度O(n)
public class MergeSort {
	public static void sort(int[] arr) {
		if(arr==null||arr.length<2) {
			return;
		}
		sortProcess(arr, 0, arr.length-1);
	}
	public static void sortProcess(int[] arr,int l,int r) {
		if (l==r) {
			return;
		}
		int mid=(l+r)/2;
		sortProcess(arr, l, mid);
		sortProcess(arr, mid+1,r);
		merge(arr, l, mid, r);
		
	}
	public static void merge(int[] arr,int l,int mid,int r) {
		int help[]=new int[r-l+1];
		int i=0;
		int p1=l;
		int p2=mid+1;
		while (p1<=mid&&p2<=r) {
			help[i++]=arr[p1]<arr[p2]?arr[p1++]:arr[p2++];
		}
		while (p1<=mid) {
			help[i++]=arr[p1++];
		}
		while (p2<=r) {
			help[i++]=arr[p2++];
		}
		for (int j = 0; j < help.length; j++) {
			arr[l+j]=help[j];
		}
	}
}