909. Snakes and Ladders

public class Solution
{
    private int boardSize;

    public int SnakesAndLadders(int[][] board)
    {
        boardSize = board.Length; // Get the size of the board (n x n)
        Queue<int> queue = new Queue<int>(); // A queue to perform BFS
        queue.Enqueue(1); // Start from the first square
        bool[] visited = new bool[boardSize * boardSize + 1]; // Visited array to keep track of visited squares
        visited[1] = true; // Mark the first square as visited
        int moves = 0; // Moves counter

        // Perform BFS to reach the last square
        while (queue.Count > 0)
        {
            for (int i = queue.Count; i > 0; --i)
            { // Iterate over current level
                int current = queue.Dequeue(); // Get the current square
                if (current == boardSize * boardSize) // Check if reached the end
                    return moves;

                // Explore the next 6 possible moves
                for (int k = current + 1; k <= Math.Min(current + 6, boardSize * boardSize); ++k)
                {
                    int[] position = ConvertToPosition(k); // Convert square number to board coordinates
                    int next = k; // Next square
                    // Check if there's a snake or ladder in the square
                    if (board[position[0]][position[1]] != -1)
                        next = board[position[0]][position[1]]; // Go to the new square

                    // If it's not visited, mark as visited and add to the queue
                    if (!visited[next])
                    {
                        visited[next] = true;
                        queue.Enqueue(next);
                    }
                }
            }
            moves++; // Increment move count after finishing one level in BFS
        }
        return -1; // Return -1 if it's impossible to reach the end
    }

    // Convert the square number to board coordinates (i, j)
    private int[] ConvertToPosition(int squareNum)
    {
        int row = (squareNum - 1) / boardSize;
        int col = (squareNum - 1) % boardSize;
        // In even rows (from the top), the order is right to left
        if (row % 2 == 1)
            col = boardSize - 1 - col;

        // Convert row to the actual row in the board from the bottom
        return new int[] { boardSize - 1 - row, col };
    }
}