LeetCode Challenge Day 80 — 2211. Count Collisions on a Road

Hey folks 👋

This is Day 80 of my LeetCode streak 🚀
Today’s problem is 2211. Count Collisions on a Road — a clever simulation-based problem that becomes surprisingly simple once you observe how cars behave at the boundaries.

📌 Problem Statement

You are given a string directions where each character represents the movement of a car:

• 'L' → moves left
• 'R' → moves right
• 'S' → stationary

Cars move at the same speed. When a collision happens:

• Both cars stop permanently.
• Collisions with opposite directions add 2 to the count.
• Collisions with stationary or already stopped cars add 1.

Return the total number of collisions that will occur.

💡 Intuition

Some cars will never collide, no matter what:

• Leading 'L' cars at the left keep moving outward and escape.
• Trailing 'R' cars at the right also escape outward.

Once these harmless cars are ignored, every remaining 'L' or 'R' in the middle zone is forced to collide with another car or obstacle.

Thus:

Remove leading 'L' and trailing 'R'.
Count how many 'L' or 'R' remain.
Each such moving car contributes exactly 1 collision.

This avoids full simulation and reduces the problem to a substring and counting task.

🔑 Approach

1. Use two pointers to skip:
   • All starting 'L' cars
   • All ending 'R' cars
2. In the remaining substring directions[i…j], every 'L' or 'R' will collide.
3. Count non-'S' characters (i.e., 'L' and 'R').
4. That count equals the total number of collisions.

This is optimal and avoids unnecessary simulation.

⏱️ Complexity Analysis

• Time Complexity:
  ( O(n) ) —
  We scan the string at most twice.

• Space Complexity:
  ( O(1) ) —
  Uses only a few pointer variables.

🧑‍💻 Code (JavaScript)

/**
 * @param {string} directions
 * @return {number}
 */
var countCollisions = function(directions) {
    const n = directions.length;
    let i = 0, j = n - 1;

    // Skip leading 'L' cars (they escape left)
    while (i < n && directions[i] === 'L') i++;

    // Skip trailing 'R' cars (they escape right)
    while (j >= 0 && directions[j] === 'R') j--;

    if (i > j) return 0; // No collision zone

    let collisions = 0;

    // Count all moving cars in the collision segment
    for (let k = i; k <= j; k++) {
        if (directions[k] !== 'S') collisions++;
    }

    return collisions;
};

🧩 Example Walkthrough

Input:

"RLRSLL"

Process:

Initial directions: R L R S L L Skip none (no leading L or trailing R) Middle segment: whole string Count of moving cars (R/L): 5 → Total collisions = 5

Output:

5

🎯 Reflection

This problem rewards pattern recognition over brute-force simulation.
The two key insights are:

• Cars moving outward from the edges never collide

• All middle cars that move must eventually collide

Once this is clear, the solution becomes a simple counting problem.
A great example of how breaking a problem into logical zones simplifies everything!

That’s it for Day 80 of my LeetCode challenge 💪
See you in the next one!

Happy Coding 👨‍💻