Merkkijono

import java.util.Scanner;

public class MerkkijonoTehtava {
    
    
    public static void main(String[] args) {
        
        StringBuilder input = new StringBuilder(new Scanner(System.in).nextLine());
        
        boolean done = false;
        
        while (!done && input.length() > 1) {
            
            done = true;
            char prevChar = input.charAt(0);
            int startIndex = -1;
            int count = 1;
            
            for (int i = 1; i < input.length(); i++) {
                
                if (input.charAt(i) == prevChar) {
                    
                    if (startIndex == -1) {
                        
                        startIndex = i - 1;
                        done = false;
                        
                        count = 2;
                        
                    } else {
                        
                        count++;
                    }
                    
                } else if (startIndex != -1) {
                    
                    input = input.delete(startIndex, startIndex + count);
                    
                    startIndex = -1;
                    
                    break;
                }
                
                prevChar = input.charAt(i);
            }
            
            if (startIndex != -1) {
                
                input = input.delete(startIndex, startIndex + count);
            }
            
        }
                
        System.out.println(input);
        
    }

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED