void __merge(int arr[], int l, int mid, int r) {
	int a = r - l + 1;
	int *aux = new int[a];
	for (int i = l; i <= r; i++)
		aux[i - l] = arr[i];

	int i = l, j = mid + 1;
	for (int k = l; k <= r; k++) {
		if (i > mid){
			arr[k] = aux[j - l]; 
			j++;
		}
		else if (j > r) { 
			arr[k] = aux[i - l];  
			i++; }
		else if (aux[i - l] < aux[j - l]) { 
			arr[k] = aux[i - l]; 
			i++; }
		else { 
			arr[k] = aux[j - l]; 
			j++; }
	}
	delete aux;
}
void __mergeSort(int arr[], int l, int r) { // 遞迴呼叫
	if (l >= r)
		return;
	int mid = (l + r) / 2;
	__mergeSort(arr, l, mid);
	__mergeSort(arr, mid+1, r);
	if( arr[mid] > arr[mid+1]) //   減少不必要的合併
		__merge(arr, l, mid, r);
}
void mergeSort(int arr[], int n) {
	__mergeSort(arr, 0, n - 1);
}
 

自底向上：

void mergeSortBU(int arr[], int n) {
	for (int sz = 1; sz <= n; sz += sz)
		for (int i = 0; i + sz < n; i += sz + sz)
			__merge(arr, i, i + sz - 1, min(i + sz + sz - 1, n - 1));
}