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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;
public class Main {
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    String str = sc.nextLine();
    List<String> arr = new ArrayList<String>();
    List<String> arr2 = new ArrayList<String>();
    int countx = 0;  
    int county = 0;  
    for(int i = 0; i < str.length(); i++) {
      String tmp = String.valueOf(str.charAt(i));
      arr2.add(tmp);
      if(arr.contains(tmp) == false){
        arr.add(tmp);
      }
    }
    if(arr.size() > 2){
      System.out.println("No");
      return;
    }
    String x = arr2.get(0);
    String y = arr2.get(1);
   
    for(int i = 0; i < str.length(); i++) {
      if(x.equals(String.valueOf(str.charAt(i)))){
      	countx++;
      }else{
        county++;
      }
    }
    if(countx != 2 || county != 2){     
		System.out.println("No");
      	return;
    }
	System.out.println("Yes");
  }
}