I've "solved" Project Euler Question 4 and I was wondering if I could make my answer more efficient (I'm using Project Euler to help me learn Haskell).

The problem reads:

Find the largest palindrome made from the product of two 3-digit numbers

Here is my solution

getMaxPalindrome = maximum[x*y | x<-[100..999], y<-[100..999], reverse(show(x*y)) ==show(x*y)]

All suggestions for improvement are appreciated!


First, since * is commutative, you can save 1/2 of your computation if you restrict yourself to cases where x >= y:

[ x*y | x<-[100..999], y<-[100..x], ... ]

Second, if you could generate all the products in a non-increasing list, you'd just be searching for the first element of such a list satisfying the predicate, which would also speed the search very much. See data-ordlist package, which implements many useful functions on sorted lists, in particular in your case you'll probably need unionBy or unionAllBy


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.