RecamansSequence.cs

usingSystem.Collections.Generic;
usingSystem.Numerics;

namespaceAlgorithms.Sequences;

/// <summary>
/// <para>
/// Recaman's sequence. a(0) = 0; for n > 0, a(n) = a(n-1) - n if nonnegative and not already in the sequence, otherwise a(n) = a(n-1) + n.
/// </para>
/// <para>
/// Wikipedia: https://en.wikipedia.org/wiki/Recam%C3%A1n%27s_sequence.
/// </para>
/// <para>
/// OEIS: http://oeis.org/A005132.
/// </para>
/// </summary>
publicclassRecamansSequence:ISequence
{
/// <summary>
/// Gets Recaman's sequence.
/// </summary>
publicIEnumerable<BigInteger>Sequence
{
get
{
yieldreturn0;
varelements=newHashSet<BigInteger>{0};
varprevious=0;
vari=1;

while(true)
{
varcurrent=previous-i;
if(current<0||elements.Contains(current))
{
current=previous+i;
}

yieldreturncurrent;
previous=current;
elements.Add(current);
i++;
}
}
}
}