909. Snakes And Ladders - Explanation

class Solution:
    def snakesAndLadders(self, board: List[List[int]]) -> int:
        n = len(board)

        def intToPos(square):
            r = (square - 1) // n
            c = (square - 1) % n
            if r % 2 == 1:
                c = n - 1 - c
            r = n - 1 - r
            return r, c

        q = deque([(1, 0)])
        visit = set()

        while q:
            square, moves = q.popleft()

            for i in range(1, 7):
                nextSquare = square + i
                r, c = intToPos(nextSquare)
                if board[r][c] != -1:
                    nextSquare = board[r][c]

                if nextSquare == n * n:
                    return moves + 1

                if nextSquare not in visit:
                    visit.add(nextSquare)
                    q.append((nextSquare, moves + 1))

        return -1

class Solution:
    def snakesAndLadders(self, board: List[List[int]]) -> int:
        n = len(board)

        def intToPos(square):
            r = (square - 1) // n
            c = (square - 1) % n
            if r % 2 == 1:
                c = n - 1 - c
            r = n - 1 - r
            return r, c

        dist = [-1] * (n * n + 1)
        q = deque([1])
        dist[1] = 0

        while q:
            square = q.popleft()

            for i in range(1, 7):
                nextSquare = square + i
                if nextSquare > n * n:
                    break

                r, c = intToPos(nextSquare)
                if board[r][c] != -1:
                    nextSquare = board[r][c]

                if dist[nextSquare] == -1:
                    dist[nextSquare] = dist[square] + 1
                    if nextSquare == n * n:
                        return dist[nextSquare]
                    q.append(nextSquare)

        return -1

class Solution:
    def snakesAndLadders(self, board: List[List[int]]) -> int:
        n = len(board)

        def intToPos(square):
            r = (square - 1) // n
            c = (square - 1) % n
            if r % 2 == 1:
                c = n - 1 - c
            r = n - 1 - r
            return r, c

        q = deque([1])
        board[n - 1][0] = 0
        moves = 0

        while q:
            for _ in range(len(q)):
                square = q.popleft()

                for i in range(1, 7):
                    nextSquare = square + i
                    if nextSquare > n * n:
                        break

                    r, c = intToPos(nextSquare)
                    if board[r][c] != -1:
                        nextSquare = board[r][c]

                    if board[r][c] != 0:
                        if nextSquare == n * n:
                            return moves + 1
                        q.append(nextSquare)
                        board[r][c] = 0
            moves += 1

        return -1