LeetCode Entry

1254. Number of Closed Islands

25.10.2026 medium 2026 kotlin rust

Count 0-islands surrounded by 1

1254. Number of Closed Islands medium substack youtube

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Problem TLDR

Count 0-islands surrounded by 1

Intuition

Do DFS from zeros. Mark visited. Entire island drops if it near the border.

Approach

• can modify visited with 1

Complexity

• Time complexity: \(O(n)\)

• Space complexity: \(O(1)\)

Code

    fun closedIsland(g: Array<IntArray>) = run {
        fun dfs(y: Int, x: Int): Int =
            if (y !in g.indices||x !in g[0].indices) 0 else if (g[y][x]>0) 1
            else (g[y][x]++*0+dfs(y+1,x)*dfs(y-1,x)*dfs(y,x+1)*dfs(y,x-1))
        g.indices.sumOf{y->g[0].indices.count{x->g[y][x]<1&&dfs(y,x)>0}}
    }

    pub fn closed_island(mut g: Vec<Vec<i32>>) -> i32 {
        fn f(g: &mut Vec<Vec<i32>>, y: usize, x: usize) -> i32 {
            if y >= g.len() || x >= g[0].len() { 0 } else if g[y][x] > 0 { 1 }
            else { g[y][x] = 1; f(g, y + 1, x) & f(g, y-1, x) & f(g, y, x + 1) & f(g, y, x-1) }
        }
        (0..g.len()).map(|y| (0..g[0].len()).filter(|&x| g[y][x] < 1 && f(&mut g, y, x) > 0).count() as i32).sum()
    }