Submission #707987

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Source Code Expand

let rec tabulate n f = 
  let rec loop acc m =
    if m < n then loop (f m :: acc) (m + 1)
    else List.rev acc in
  loop [] 0

let rec power ( * ) one m = function
  | 0 -> one
  | n when n mod 2 = 0 ->
      power ( * ) one (m * m) (n / 2)
  | n ->
      m * power ( * ) one m (n - 1)

let n = 1000000007
let ( * ) x y = (x * y) mod n
let fact m = List.fold_left ( * ) 1 (tabulate m (fun i -> (i + 1) mod n))

let () = Scanf.scanf "%d %d" (fun w h ->
  Printf.printf "%d\n" @@ 
    fact (h + w - 2) * power ( * ) 1 (fact (h - 1) * fact (w - 1)) (n - 2))

Submission Info

Judge Result