Project Euler – Problem 52 Solution

Yan Cui

I help clients go faster for less using serverless technologies.

Problem

It can be seen that the number, 125874, and its double, 251748, contain exactly the same digits, but in a different order.

Find the smallest positive integer, x, such that 2x, 3x, 4x, 5x, and 6x, contain the same digits.

Solution

// check if two numbers are permutations of the same set of digits
let isPermutation number1 number2 =
    let str1 = number1.ToString().ToCharArray() |> Array.sort
    let str2 = number2.ToString().ToCharArray() |> Array.sort

    if str1.Length <> str2.Length then false
    else Array.forall2 (fun x y -> x = y) str1 str2

let answer =
    Seq.unfold (fun state -> Some(state, state + 1)) 1
    |> Seq.filter (fun n -> 
        [2..6] |> List.map (fun x -> x * n) |> List.forall (fun xn -> isPermutation xn n))
    |> Seq.head

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