Count sub islands

Array Depth-First Search Breadth-First Search Union-Find Matrix

73.0% acceptance

Feb 25, 2026

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You are given two m x n binary matrices grid1 and grid2 containing only 0's (representing water) and 1's (representing land). An island is a group of 1's connected 4-directionally (horizontal or vertical). Any cells outside of the grid are considered water cells. An island in grid2 is considered a sub-island if there is an island in grid1 that contains all the cells that make up this island in grid2. Return the number of islands in grid2 that are considered sub-islands.

Solution

Rust

Time O(n²)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn count_sub_islands(grid1: Vec<Vec<i32>>, mut grid2: Vec<Vec<i32>>) -> i32 {
    let m = grid2.len();
    let n = grid2[0].len();
    let mut count = 0;
    for i in 0..m {
      for j in 0..n {
        if grid2[i][j] == 1 {
          let mut is_sub = true;
          Self::dfs(&grid1, &mut grid2, i, j, &mut is_sub);
          if is_sub {
            count += 1;
          }
        }
      }
    }
    count
  }

  fn dfs(grid1: &[Vec<i32>], grid2: &mut Vec<Vec<i32>>, i: usize, j: usize, is_sub: &mut bool) {
    if i >= grid2.len() || j >= grid2[0].len() || grid2[i][j] == 0 {
      return;
    }
    if grid1[i][j] == 0 {
      *is_sub = false;
    }
    grid2[i][j] = 0;
    Self::dfs(grid1, grid2, i + 1, j, is_sub);
    if i > 0 { Self::dfs(grid1, grid2, i - 1, j, is_sub); }
    Self::dfs(grid1, grid2, i, j + 1, is_sub);
    if j > 0 { Self::dfs(grid1, grid2, i, j - 1, is_sub); }
  }
}