Mergesort?

void MergeSort(int[] arr, int left, int right)

if (left < right)

int mid = (left + right) / 2;

MergeSort(arr, left, mid);

MergeSort(arr, mid + 1, right);

Follow on:

Merge(arr, left, mid, right);

void Merge(int[] arr, int left, int mid, int right)

int n1 = mid - left + 1;

int n2 = right - mid;

int[] L = new int[n1];

int[] R = new int[n2];

Array.Copy(arr, left, L, 0, n1);

Array.Copy(arr, mid + 1, R, 0, n2);

int i = 0, j = 0, k = left;

while (i < n1 && j < n2)

arr[k++] = (L[i] <= R[j]) ? L[i++] : R[j++];

while (i < n1) arr[k++] = L[i++];

while (j < n2) arr[k++] = R[j++];

Explanation:

Divide array, sort left & right halves, then merge sorted halves.