Merge Sort

function mergeSort(var a : array of integer; minIndex, maxIndex : integer);
{Pre: 	a defined
		maxIndex>=minIndex;
		minIndex>=0;
		a[minIndex..maxIndex] defined
 Post:	for each i into minIndex..maxIndex, a[maxIndex] >= a[i] 
}
var center : integer; {center of the array}
begin
	if minIndex > maxIndex 	                    {O(1)}
	then begin
		center := (minIndex + maxIndex) div 2;  {O(1)}
		mergeSort(a, minIndex, center);         {T(n/2)}
		mergeSort(a, center+1, maxIndex);       {T(n/2)}
		merge(a, minIndex, center, maxIndex);   {O(n)}
	end
end;