count_collisions_on_a_road.py

# There are n cars on an infinitely long road. The cars are numbered from 0 to n - 1 from left to right and each car is present at a unique point.

# You are given a 0-indexed string directions of length n. directions[i] can be either 'L', 'R', or 'S' denoting whether the ith car is moving towards 
# the left, towards the right, or staying at its current point respectively. Each moving car has the same speed.

# The number of collisions can be calculated as follows:

# When two cars moving in opposite directions collide with each other, the number of collisions increases by 2.
# When a moving car collides with a stationary car, the number of collisions increases by 1.
# After a collision, the cars involved can no longer move and will stay at the point where they collided. Other than that, cars cannot change their 
# state or direction of motion.

# Return the total number of collisions that will happen on the road.

 

# Example 1:

# Input: directions = "RLRSLL"
# Output: 5
# Explanation:
# The collisions that will happen on the road are:
# - Cars 0 and 1 will collide with each other. Since they are moving in opposite directions, the number of collisions becomes 0 + 2 = 2.
# - Cars 2 and 3 will collide with each other. Since car 3 is stationary, the number of collisions becomes 2 + 1 = 3.
# - Cars 3 and 4 will collide with each other. Since car 3 is stationary, the number of collisions becomes 3 + 1 = 4.
# - Cars 4 and 5 will collide with each other. After car 4 collides with car 3, it will stay at the point of collision and get hit by car 5. 
# The number of collisions becomes 4 + 1 = 5.
# Thus, the total number of collisions that will happen on the road is 5. 
# Example 2:

# Input: directions = "LLRR"
# Output: 0
# Explanation:
# No cars will collide with each other. Thus, the total number of collisions that will happen on the road is 0.

class Solution:
    def countCollisions(self, directions: str) -> int:
        count = 0
        # this will indicate either a stationary or collided car behind
        carBehind = False
        # this will indicate the number of cars moving right from behind
        movingCarsBehind = 0
        
        for i in range(len(directions)):
            if directions[i] == 'R':
                movingCarsBehind += 1
            elif directions[i] == 'S':
                carBehind = True
                if movingCarsBehind != 0:
                    count += movingCarsBehind
                    movingCarsBehind = 0
            else:
                if movingCarsBehind != 0:
                    # moving car hits this car and this car is left behind
                    carBehind = True
                    count += 2
                    count += (movingCarsBehind - 1)
                    movingCarsBehind = 0
                elif carBehind:
                    # this car hits stationary car behind and becomes stationary
                    carBehind = True
                    count += 1
        
        return count