LeetCode Entry

2211. Count Collisions on a Road

04.12.2025 medium 2025 kotlin rust

Collisions R vs L

2211. Count Collisions on a Road medium blog post substack youtube

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Problem TLDR

Collisions R vs L #medium

Intuition

Do separate pass for R, then backwards pass for L. Drop counter every time opposite meets.

Approach

• notice, only the prefix L and suffix R are excluded.

Complexity

• Time complexity: \(O(n)\)

• Space complexity: \(O(n)\), can be O(1)

Code

// 42ms
    fun countCollisions(d: String) =
        d.trimStart('L').trimEnd('R').count { it != 'S' }

// 0ms
    pub fn count_collisions(d: String) -> i32 {
       d.trim_start_matches('L').trim_end_matches('R').bytes()
       .filter(|&b| b != b'S').count() as _
    }