Wed Dec 2 16:36:07 EST 2009

```Hi,

I am trying to solve this problem: https://www.spoj.pl/problems/LASTDIG/
It is very simple. Given a and b, return the last digit of a^b. b
could be large, so I used logarithmic exponentiation and
wrote/submitted the code below for this problem:

----------------------------------------------------------------------
lastdigit :: Int -> Int -> Int -> Int
lastdigit 0 0 _ = 1
lastdigit a b c | even b = lastdigit ( (a*a) `rem` 10 ) (b `quot` 2) c
| b == 1 = (a*c) `rem` 10
| otherwise = lastdigit ( (a*a) `rem` 10 ) (b `quot` 2) (a*c)

doit :: [Char] -> Int
doit line = lastdigit (read \$ head \$ words line) (read \$ last \$ words line) 1

main = do
n <- getLine
inputs <- sequence \$ take (read n) \$ repeat getLine
let slist = map doit inputs
mapM_ (putStrLn.show) slist
-------------------------------------------------------------------

As n in main is at most 30, I thought this would easily run in 1
second, but I get a time limit exceeded error on the site. Can someone
tell me where my code is taking too much time?