核心思路：
先分段排序，再归并，再复制到原来数组中。
优质代码： 
 

void merge(int arr[], int L, int M, int R);
void process(int arr[], int L, int R);
void mergeSort(int arr[], int sz) {
	if (arr == NULL || sz < 2)
		return;
	process(arr, 0, sz - 1);
}
void process(int arr[], int L,int R) {
	if (L == R)
		return;
	int M = L + (R - L) / 2;
	process(arr, L, M);
	process(arr, M + 1, R);
	merge(arr, L, M, R);
}
void merge(int arr[], int L, int M, int R) {
	int help[]= new int[R - L + 1];
	int i = 0;
	int p1 = L, p2 = M + 1;
	while (p1 <= L && p2 <= R) 
		help[i++] = arr[p1] > arr[p2] ? arr[p2++] : arr[p1++];
	while (p1 <= L)
		help[i++] = arr[p1++];
	while (p2 <= R)
		help[i++] = arr[p2++];
	for (int j = 0;j < i;j++)
		arr[j+L] = help[j];
}