Project Euler - Problem #4

Description

• 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.

Solution (Improved)

The improvements are minor, but include a smaller corelation matrix amd more concise syntax.

let corrMatrix start last =
     seq {for i in start..last do
          for j in start..last do
               if i<  j then yield (i*j)}

 let reverse (s:string) = new string(s |> Seq.toArray |> Array.rev)

let palindromes =
     (corrMatrix 100 999) |> Seq.filter (fun x -> string x = reverse(string x)) |> Seq.max

Solution (Original)

//function for reversing a string
let reverse (s:string) = new string(s |> Seq.toArray |> Array.rev)

//function to multiple each number in the series by itself, similar to a correlation matrix
let corrMatrix start last =
     seq {for i in start..last do
          for j in start..last do
               yield (i*j)}

//finds the maximum number that is the same forward as backwards
let palindromes =
     let lst = corrMatrix 100 999
     let comparison = lst |> Seq.filter (fun x -> string x = reverse(string x))
     Seq.max comparison