909. Snakes and Ladders

class Solution {
    public int snakesAndLadders(int[][] board) {
        if (board == null || board.length == 0) return 0;
        int n = board.length;
        boolean[] used = new boolean[n * n + 1];
        
        Queue<Integer> q = new ArrayDeque<>();
        q.offer(1);
        
        int moves = 0;
        int min = n * n;
        while (!q.isEmpty()) {
            int size = q.size();
            
            for (int i = 0; i < size; i++) {
                int cur = q.poll();
                if (cur == n * n) {
                    min = Math.min(min, moves);
                }
              
                for (int j = 1; j <= 6; j++) { 
                    if (j + cur > n * n) break;             
                    
                    if (!used[j + cur]) {
                        used[j + cur] = true;
                        int row = n - (j + cur - 1) / n - 1;

                        int col = ((n - row) % 2 != 0) ? (j + cur - 1) % n : n - (j + cur - 1) % n - 1;
                                              
                        if (board[row][col] == cur) continue;
                        if (board[row][col] == -1) {
                            q.offer(j + cur);
                        } else {
                            q.offer(board[row][col]);
                        }
                    }
                }
            }
            
            moves++;
        }
        
        return min == n * n ? -1 : min;
    }
}