LeetCode 2211: Count Collisions on a Road Solution

Master LeetCode problem 2211 (Count Collisions on a Road), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

2211. Count Collisions on a Road

Medium

String Stack Simulation

Problem Explanation

Explanation:

To solve this problem, we can iterate through the cars from left to right. For each car, we check if it is moving left or right. If it is moving right, we store its position. If it is moving left, we check if there are any cars that were moving right and have not collided yet. If there are, we calculate the collisions and update the total count. We repeat this process for all cars.

Algorithm:

Initialize variables collisions to 0 and an empty set carsMovingRight.
Iterate through the cars:
- If car i is moving right, add its position to carsMovingRight.
- If car i is moving left:
  - Check for any cars moving right that haven't collided yet.
  - Update collisions accordingly.
Return the total collisions.

Time Complexity: O(n) where n is the number of cars.

Space Complexity: O(n) for storing the set of cars moving right.

Solution Code

class Solution {
    public int countCollisions(String directions) {
        int collisions = 0;
        Set<Integer> carsMovingRight = new HashSet<>();
        
        for (int i = 0; i < directions.length(); i++) {
            if (directions.charAt(i) == 'R') {
                carsMovingRight.add(i);
            } else if (directions.charAt(i) == 'L') {
                for (int pos : carsMovingRight) {
                    if (pos < i) {
                        collisions += 2;
                    } else {
                        collisions++;
                    }
                }
            }
        }
        
        return collisions;
    }
}

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