c#归并排序

void MergeSort(int[] arr)
{
    int l = 0;
    int r = arr.Length-1;
    MergeSortSub(arr,l,r);
}
// 1,2
void MergeSortSub(int[] arr,int left,int right)
{
    if(left == right) return;
    int mid = left + ((right - left) >> 1);
    MergeSortSub(arr, left, mid);
    MergeSortSub(arr, mid+1, right);
    Merge(arr,left,mid,right);
}
// right也要传
void Merge(int[] arr,int left,int mid,int right){
    int[] help = new int[right-left+1];//*
    int i=0;
    int p1 = left;
    int p2 = mid+1;
    while(p1 <= mid && p2 <= right)
    {
        help[i++] = arr[p1] <= arr[p2] ? arr[p1++] : arr[p2++];
    }
    while(p1 <= mid)
    {
        help[i++] = arr[p1++];
    }
    while(p2 <= right)
    {
        help[i++] = arr[p2++];
    }
    for ( i = 0; i < help.Length; i++)
    {
        arr[left+i] = help[i]; // **
    }
}