TURN EACH CAR INTO THE TIME IT WOULD REACH THE TARGET ALONE, CLOSEST CAR FIRST. A CAR THAT ARRIVES AFTER THE FLEET AHEAD IS TOO SLOW TO CATCH IT, SO IT STARTS A NEW FLEET. ONE THAT ARRIVES SOONER JOINS THE FLEET AHEAD. THE STACK OF FLEET TIMES STAYS INCREASING, AND ITS SIZE IS THE ANSWER.

BLUE IS THE CAR BEING CHECKED. TAN TILES ARE FLEET LEADERS ON THE STACK. GREEN IS A FLEET JUST CAUGHT, AND DIM CARS HAVE BEEN ABSORBED. THE TIME ROW FILLS AS CARS ARE REACHED, AND THE SHELF HOLDS ONE TIME PER FLEET. AT THE END THE LEADERS GOLD.

1def car_fleet(target, position, speed):2    stack = []3    zipped = list(zip(position, speed))4    zipped.sort(reverse=True)5    for i in range(len(zipped)):6        t = (target - zipped[i][0]) / zipped[i][1]7        if not stack:8            stack.append(t)9        elif t > stack[-1]:10            stack.append(t)11    return len(stack)