(Leetcode) Snakes and Ladders

Question

This is a Leetcode problem -

On an \$N\$ x \$N\$ board, the numbers from 1 to N * N are written boustrophedonically (starting from the bottom left of the board), and alternating direction each row. For example, for a 6 x 6 board, the numbers are written as follows -

You start on square 1 of the board (which is always in the last row and first column). Each move, starting from square x, consists of the following -

• You choose a destination square S with number x+1, x+2, x+3, x+4, x+5, or x+6, provided this number is <= N * N.
  • (This choice simulates the result of a standard 6-sided die roll, ie., there are always at most 6 destinations, regardless of the size of the board.)
• If S has a snake or ladder, you move to the destination of that snake or ladder. Otherwise, you move to S.

A board square on row r and column c has a "snake or ladder" if board[r][c] != -1. The destination of that snake or ladder is board[r][c].

Note that you only take a snake or ladder at most once per move; if the destination to a snake or ladder is the start of another snake or ladder, you do not continue moving. (For example, if the board is [[4,-1],[-1,3]], and on the first move your destination square is 2, then you finish your first move at 3 because you do not continue moving to 4.)

Return the least number of moves required to reach square N * N. If it is not possible, return -1.

Note -

• 2 <= board.length = board[0].length <= 20
• board[i][j] is between 1 and N * N or is equal to -1.
• The board square with number 1 has no snake or ladder.
• The board square with number N * N has no snake or ladder.

Example 1 -

Input: [
[-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]]

Output: 4

"""
Explanation -

At the beginning, you start at square 1 [at row 5, column 0].

You decide to move to square 2, and must take the ladder to square 15.

You then decide to move to square 17 (row 3, column 5), and must take the snake to square 13.

You then decide to move to square 14, and must take the ladder to square 35.

You then decide to move to square 36, ending the game.

It can be shown that you need at least 4 moves to reach the N*N-th square, so the answer is 4.

"""

I would like to have a performance review of my solution and would also like to know whether I could make it more efficient.

Here is my solution to this challenge (in Python 3) -

# Uses Breadth First Search (BFS)

def snakes_and_ladders(board):

    """
    :type board: List[List[int]]
    :rtype: int
    """

    board_2 = [0]
    rows, cols = len(board), len(board[0])
    row = rows - 1

    while row >= 0:
        for col in range(cols):
            board_2.append(board[row][col])
        row -= 1
        if row >= 0:
            for col in range(cols - 1, -1, -1):
                board_2.append(board[row][col])
            row -= 1

    visited = [0 for i in range(len(board_2))]
    stack = collections.deque()
    stack.append([1,0])
    while stack:
        current_index, current_dist = stack.popleft()
        for i in range(1,7):
            next_index = min(rows * cols, current_index + i)
            if board_2[next_index] != -1:
                next_index = board_2[next_index]
            if next_index == rows * cols:
                return current_dist + 1
            if visited[next_index] == 0:
                visited[next_index] = 1
                stack.append([next_index, current_dist + 1])

    return -1

Answer 1

Type Hinting

You were halfway there! Good job having the type of values accepted and returned in the method docstring. Now, you can use type hints to show in the header of the method what values are accepted and returned, as follows

from typing import List

def snakes_and_ladders(board: List[List[int]]) -> int:

Consistent Spacing

This

for i in range(1,7):

should be this

for i in range(1, 7):

You have good spacing in the rest of your code, but you should stay consistent and apply this spacing everywhere.

The same for this line

stack.append([1,0]) -> stack.append([1, 0])

Magic Numbers

We're coming back to this line again

for i in range(1, 7):

What is 7 supposed to represent? The max number of rows or columns? What if you have to change it later on to apply to smaller/bigger snakes and ladders boards? I would advise using a variable to hold this value, naming it accordingly.

List Comprehension

You use lots of loops with one line in them. Particularly when appending to lists. Luckily, you can add two lists together and it will merge them.

From this

for col in range(cols):
    board_2.append(board[row][col])

to this

board_2 += [board[row][col] for col in range(cols)]

You can do the same for the next loop a couple lines down

From this

for col in range(cols - 1, -1, -1):
    board_2.append(board[row][col])

to this

board_2 += [board[row][col] for col in range(cols - 1, -1, -1)]