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+# 909. Snakes and Ladders
+
+"""
+you are given an 'n x n' integer matrix 'board' where the cells are all
+labeled from 1 to 'n^2' in 'boustropheden style' starting from bottom left of
+the board, and alternating direction each row. a board square on row 'r' and
+column 'c' has a snake of a ladder if 'board[r][c] != -1'. the destination of
+that snake or ladder is 'board[r][c]'. squares 1 and 'n^2' are not the
+starting points of any snake or ladder. note that you only take a snake or a
+ladder at most once per dice roll. if the destination to a snake or ladder is
+the start of another snake or ladder, you do not follow the subsequent snake
+or ladder.
+"""
+
+
+class Solution(object):
+ def snakesAndLadders(self, board):
+ """
+ :type board: List[List[int]]
+ :rtype: int
+ """
+ n = len(board)
+
+ def coordinates(num):
+ row, col = (num - 1) // n, (num - 1) % n
+ col = n - 1 if row % 2 == 1 else col
+ row = n - 1 - row
+ return row, col
+
+ vis, queue = set(), [(1, 0)]
+ vis.add(1)
+ while queue:
+ curr, moves = queue.pop(0)
+ if curr == n**2:
+ return moves
+ for i in range(1, 7):
+ next = curr + i
+ if next > n**2:
+ continue
+ row, col = coordinates(next)
+ if board[row][col] != -1:
+ next = board[row][col]
+ if next == n**2:
+ return moves + 1
+ if next not in vis:
+ vis.add(next)
+ queue.append((next, moves + 1))
+ return -1
+
+
+if __name__ == "__main__":
+ obj = Solution()
+ print(
+ obj.snakesAndLadders(
+ board=[
+ [-1, -1, -1, -1, -1, -1],
+ [-1, -1, -1, -1, -1, -1],
+ [-1, -1, -1, -1, -1, -1],
+ [-1, 35, -1, -1, 13, -1],
+ [-1, -1, -1, -1, -1, -1],
+ [-1, 15, -1, -1, -1, -1],
+ ]
+ )
+ )
+ print(obj.snakesAndLadders(board=[[-1, -1], [-1, 3]]))