postApril 13, 2026

Count Islands (Matrix DFS)

#javascript#graph#dfs#algorithms#matrix

Hard⏱️ Logic Challenge

Given a 2D grid of '1' (land) and '0' (water), count the number of islands. An island is formed by connecting adjacent lands horizontally or vertically.

Ver solución

function numIslands(grid) {
  let count = 0;

  function dfs(r, c) {
    if (r < 0 || r >= grid.length ||
        c < 0 || c >= grid[0].length ||
        grid[r][c] !== '1') return;
    grid[r][c] = '0';  // mark visited
    dfs(r+1,c); dfs(r-1,c); dfs(r,c+1); dfs(r,c-1);
  }

  for (let r = 0; r < grid.length; r++) {
    for (let c = 0; c < grid[0].length; c++) {
      if (grid[r][c] === '1') {
        dfs(r, c);
        count++;
      }
    }
  }
  return count;
}

// When we find a '1', flood-fill the whole island with '0' (DFS).
// Each flood-fill = 1 island.