Merkkijono

str = ARGF.readline.strip
a = []
f = false
str.each_char do |c|
  if a.last == c
    f = true
  else
    a.pop if f
    if a.last != c
      f = false
      a.push c
    end
  end
end
a.pop if f
puts a.join

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