# Here your code !
def possible?(s)
    lines = []
    s.chars.reverse.find {|c|
        if c == 'R'
            lines << 2
            false
        elsif c == 'G'
            redpos = lines.find_index(2)
            lines[redpos] = 1 if redpos
            !redpos
        else
            wgpos = lines.find_index {|v| v < 2}
            lines[wgpos] = 0 if wgpos
            lines.sort if wgpos > 0
            !wgpos
        end
    }.nil?
end