Recursionmedium
0:00.0

The number of derangements of nn items (permutations where no element is in its original position) satisfies: D(n)=(n1)[D(n1)+D(n2)]D(n) = (n-1)[D(n-1) + D(n-2)] with D(0)=1,D(1)=0D(0) = 1, D(1) = 0. Compute D(3)D(3).