Project Euler #004

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 99. Find the largest palindrome made from the product of two 3-digit numbers.

let rec reverse (s:string) = 
    if s.Length = 1 then s
    else (reverse (s.Substring 1)) + (s.[0] |> string)

let is_palindrome n = (n |> string) = reverse (n |> string)

[100..999] |> List.collect (fun x -> [100..999] |> List.map (fun y -> x * y))
    |> List.filter (fun x -> is_palindrome x)
    |> List.max