def dutch_flag_sort(balls):
    red = green = -1
    for blue in range(len(balls)):
        if balls[blue] == "R":
            red += 1
            balls[red], balls[blue] = balls[blue], balls[red]
            green += 1
            if green != red:
                balls[green], balls[blue] = balls[blue], balls[green]
        elif balls[blue] == "G":
            green += 1
            balls[green], balls[blue] = balls[blue], balls[green]