LeetCode Entry

3619. Count Islands With Total Value Divisible by K

26.10.2026 medium 2026 kotlin rust

Count islands sum %k in grid

3619. Count Islands With Total Value Divisible by K medium substack youtube

https://dmitrysamoylenko.com/leetcode/

Join me on Telegram

https://t.me/leetcode_daily_unstoppable/1523

Problem TLDR

Count islands sum %k in grid

Intuition

Do dfs, modifiy in-place to zeros

Approach

• Rust pattern-match Some(v @ 1..) = g.get_mut(y)

Complexity

• Time complexity: \(O(n)\)

• Space complexity: \(O(1)\)

Code

    fun countIslands(g: Array<IntArray>, k: Int)=run{
        fun d(y: Int, x: Int): Long = g.getOrNull(y)?.getOrNull(x)?.takeIf{it>0}
            ?.let { g[y][x]=0; it+d(y-1,x)+d(y+1,x)+d(y,x-1)+d(y,x+1)}?:0
        g.indices.sumOf{y->g[0].indices.count{val d=d(y,it); d>0&&d%k<1}}
    }

    pub fn count_islands(mut g: Vec<Vec<i32>>, k: i32) -> i32 {
        fn d(g: &mut [Vec<i32>], y: usize, x: usize) -> i64 {
            if let Some(v @ 1..) = g.get_mut(y).and_then(|r| r.get_mut(x)) {
                take(v) as i64 + d(g, y - 1, x) + d(g, y + 1, x) + d(g, y, x - 1) + d(g, y, x + 1)
            } else { 0 }
        }
        (0..g.len()).fold(0, |a, y| (0..g[0].len()).fold(a, |a, x| {
            let s = d(&mut g, y, x); a + (s > 0 && s % k as i64 == 0) as i32
        }))
    }