Derangement count
A derangement of n distinct items is a permutation in which no item appears in its original position. Given n (0 <= n <= 10^6) and a modulus mod (>= 2), return the number of derangements of n items, modulo mod. By convention there is exactly one derangement of 0 items (the empty arrangement) and zero derangements of 1 item. For example, 3 items have 2 derangements.
Implement
count_derangements(n: int, mod: int) → intExamples
in
[3,1000000007]out2What a strong answer looks like
State your approach and its time/space complexity out loud before you optimize. Handle the edge cases (empty input, duplicates, overflow), and say why you chose this over the brute force. Green tests are the floor, not the grade.
0:00 of about 25 min
solution.py
InputExpectedGot
[3,1000000007]2not run yetsample