Problem:
Score : 300 points

Problem Statement

Snuke has N dogs and M monkeys. He wants them to line up in a row.

As a Japanese saying goes, these dogs and monkeys are on bad terms. ("ken'en no naka", literally "the relationship of dogs and monkeys", means a relationship of mutual hatred.) Snuke is trying to reconsile them, by arranging the animals so that there are neither two adjacent dogs nor two adjacent monkeys.

How many such arrangements there are? Find the count modulo 10^9+7 (since animals cannot understand numbers larger than that).
Here, dogs and monkeys are both distinguishable. Also, two arrangements that result from reversing each other are distinguished.

Constraints

1 ≤ N,M ≤ 10^5

Input

Input is given from Standard Input in the following format:

N M

Output

Print the number of possible arrangements, modulo 10^9+7.

Sample Input 1

2 2

Sample Output 1

8

We will denote the dogs by A and B, and the monkeys by C and D. There are eight possible arrangements: ACBD, ADBC, BCAD, BDAC, CADB, CBDA, DACB and DBCA.

Sample Input 2

3 2

Sample Output 2

12

Sample Input 3

1 8

Sample Output 3

0

Sample Input 4

100000 100000

Sample Output 4

530123477

Solution in Haskell (GHC 7.10.3) (judge verdict: Accepted)

import           Control.Monad
import qualified Data.ByteString.Char8       as BS
import           Data.Maybe
import qualified Data.Vector.Unboxed.Mutable as VM

readInt = fst . fromJust . BS.readInt
readIntList = map readInt . BS.words
getInt = readInt <$> BS.getLine
getIntList = readIntList <$> BS.getLine

main = do
  [n, m] <- getIntList

  let (x, y) = if n >= m then (n, m) else (m, n)
      p = 10^9 + 7 :: Int

  d <- VM.new (x+1)
  VM.set d (0::Int)

  let fac :: Int -> IO Int
      fac x | x == 1    = return 1
            | otherwise = do
                t <- VM.read d x
                if t > 0
                  then return t
                  else do fx_1 <- fac (x-1)
                          let r = x * fx_1 `mod` p
                          VM.write d x r
                          return r

      solve :: Int -> Int -> IO Int
      solve a b | a - b >= 2 = return 0
                | a - b == 1 = do fa <- fac a
                                  fb <- fac b
                                  return $ fa * fb `mod` p
                | otherwise  = do -- a == b
                    fa <- fac a
                    return $ fa * fa `mod` p * 2 `mod` p

  ans <- solve x y
  print ans