Problem:
Score : 100 points

Problem Statement

You are given a 4-character string S consisting of uppercase English letters.
Determine if S consists of exactly two kinds of characters which both appear twice in S.

Constraints

The length of S is 4.

S consists of uppercase English letters.

Input

Input is given from Standard Input in the following format:

S

Output

If S consists of exactly two kinds of characters which both appear twice in S, print Yes; otherwise, print No.

Sample Input 1

ASSA

Sample Output 1

Yes

S consists of A and S which both appear twice in S.

Sample Input 2

STOP

Sample Output 2

No

Sample Input 3

FFEE

Sample Output 3

Yes

Sample Input 4

FREE

Sample Output 4

No

Solution in OCaml (4.02.3) (judge verdict: Accepted)

open Batteries
let explode s = List.init (String.length s) (String.get s)
let s = read_line () |> explode

let get_all_elements l =
  let rec get existed lst =
    match lst with
     [] -> existed
    | first :: rest -> if List.exists (fun a -> a = first) existed then get existed rest 
                       else get (first :: existed) rest
  in get [] l

let ans l =
  let rec p lst =
    match lst with
    | [] -> true
    | first :: rest -> if List.length (List.find_all (fun a -> a = first) s) = 2 then p rest else false
  in p l

let all = get_all_elements s
let _ = if List.length all != 2 then print_endline "No"
        else if ans all then print_endline "Yes" else print_endline "No"