Graph BFS/DFS

function numIslands(grid: string[][]): number {
  const rows = grid.length, cols = grid[0]?.length ?? 0;
  let count = 0;
  const sink = (r: number, c: number): void => {
    if (r < 0 || c < 0 || r >= rows || c >= cols || grid[r][c] !== '1') return;
    grid[r][c] = '0'; // mark visited in place
    sink(r + 1, c); sink(r - 1, c); sink(r, c + 1); sink(r, c - 1);
  };
  for (let r = 0; r < rows; r++)
    for (let c = 0; c < cols; c++)
      if (grid[r][c] === '1') { count++; sink(r, c); }
  return count;
}

def num_islands(grid)
  rows = grid.length
  cols = rows.zero? ? 0 : grid[0].length
  count = 0
  sink = lambda do |r, c|
    return if r.negative? || c.negative? || r >= rows || c >= cols || grid[r][c] != '1'

    grid[r][c] = '0' # mark visited in place
    sink.call(r + 1, c)
    sink.call(r - 1, c)
    sink.call(r, c + 1)
    sink.call(r, c - 1)
  end
  rows.times do |r|
    cols.times do |c|
      next unless grid[r][c] == '1'
      count += 1
      sink.call(r, c)
    end
  end
  count
end