Explain merge sort — divide, conquer, merge. Complexity?

split → sort left, sort right → merge two sorted lists

void merge(int[] a, int l, int m, int r) {
  int[] tmp = new int[r - l + 1];
  int i = l, j = m + 1, k = 0;
  while (i <= m && j <= r) tmp[k++] = a[i] <= a[j] ? a[i++] : a[j++];
  while (i <= m) tmp[k++] = a[i++];
  while (j <= r) tmp[k++] = a[j++];
  for (int t = 0; t < tmp.length; t++) a[l + t] = tmp[t];
}