LeetCode Entry

3665. Twisted Mirror Path Count

31.10.2026 medium 2026 kotlin rust

Count ways to move in grid with 1-mirrors

3665. Twisted Mirror Path Count medium substack youtube

https://dmitrysamoylenko.com/leetcode/

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Problem TLDR

Count ways to move in grid with 1-mirrors

Intuition

DFS DP: cache by position and direction Bottom up DP: keep previous row as direction down and previos value as direction right, swap on mirrors

Approach

• Rust allows to modify array values while iterating it

Complexity

• Time complexity: \(O(n)\)

• Space complexity: \(O(n)\)

Code

    fun uniquePaths(g:Array<IntArray>): Int {
        val d = IntArray(501); d[0] = 1; var a = 0
        for (r in g) {
            var R = 0
            for (x in r.indices) {
                a = (R + d[x]) % 1000000007
                if (r[x] < 1) { d[x] = a; R = a } else R = d[x].also { d[x] = R }
            }
        }
        return a
    }

    pub fn unique_paths(g: Vec<Vec<i32>>) -> i32 {
        let (mut d, mut a) = ([0; 501], 0); d[0] = 1;
        for r in g {
            let mut R = 0;
            for (D, v) in d.iter_mut().zip(r) {
                a = (R + *D) % 1000000007;
                (*D, R) = if v < 1 { (a, a) } else { (R, *D) }
            }
        } a
    }