NumberOfIslands.cs

public class NumberOfIslands
{
    public static int NumIslands(char[][] grid)
    {
        int numberOfIslands = 0;
        for (int i = 0; i < grid.Length; i++)
        {
            for (int j = 0; j < grid[i].Length; j++)
            {
                if (grid[i][j] == '1')
                {
                    numberOfIslands++;
                    Traverse(grid, new Coords(x: j, y: i));
                }
            }
        }

        return numberOfIslands;
    }

    private readonly struct Coords(int x, int y)
    {
        public readonly int X = x;
        public readonly int Y = y;
    }

    enum Direction
    {
        Up,
        Right,
        Down,
        Left,
    }

    private static Coords GetDirection(Coords coordinates, Direction direction) =>
        direction switch
        {
            Direction.Up => new Coords(coordinates.X, coordinates.Y - 1),
            Direction.Right => new Coords(coordinates.X + 1, coordinates.Y),
            Direction.Down => new Coords(coordinates.X, coordinates.Y - 1),
            Direction.Left => new Coords(coordinates.X - 1, coordinates.Y),
            _ => throw new NotImplementedException(),
        };

    private static void Traverse(char[][] grid, Coords coordinates)
    {
        if (
            coordinates.X < 0
            || coordinates.X >= grid[0].Length
            || coordinates.Y < 0
            || coordinates.Y >= grid.Length
        )
        {
            return;
        }

        // if we're not an island or we've seen this return buddy
        if (grid[coordinates.Y][coordinates.X] != '1')
            return;
        grid[coordinates.Y][coordinates.X] = 's';

        foreach (var direction in Enum.GetValues<Direction>())
        {
            Traverse(grid, GetDirection(coordinates, direction));
        }
    }
}