private static int DeterministicSelection(int[] arr, int begin, int end, int i)
{
if (end - begin <= 5)
{
int[] b = arr.SubArray(begin, end);
Array.Sort(b);
return(b[i - 1]);
}
int[] medians = new int[(arr.Length + 4) / 5];
for (int k = 0; k < medians.Length; k++)
{
int[] a = new int[Math.Min(5, arr.Length - 5 * k)];
for (int p = 5 * k; p < 5 * (k + 1) && p < arr.Length; p++)
{
a[p - 5 * k] = arr[p];
}
Array.Sort(a);
medians[k] = a[Median(a)];
}
int median = DeterministicSelection(medians, 0, medians.Length - 1, medians.Length / 2);
int medianIndex = begin;
for (int k = begin; k <= end; k++)
{
if (arr[k] == median)
{
medianIndex = k;
}
}
int pivotIndex = Sort.Partition(arr, 0, arr.Length - 1, medianIndex);
int order = pivotIndex + 1 - begin;
if (order == i)
{
return(arr[pivotIndex]);
}
else if (order < i)
{
return(DeterministicSelection(arr, pivotIndex + 1, end, i - order));
}
return(DeterministicSelection(arr, begin, pivotIndex - 1, i));
}
private static int RandomizedSelect(int[] arr, int begin, int end, int i)
{
if (begin == end)
{
return(arr[begin]);
}
int ind = random.Next(begin, end + 1);
int pivot = Sort.Partition(arr, begin, end, ind);
int order = pivot - begin + 1;
if (order == i)
{
return(arr[pivot]);
}
else if (order < i)
{
return(RandomizedSelect(arr, pivot + 1, end, i - order));
}
return(RandomizedSelect(arr, begin, pivot - 1, i));
}