Let's play the minesweeper game (Wikipedia, online game)!

You are given a 2D char matrix representing the game board. 'M' represents an unrevealed mine, 'E' represents an unrevealed empty square, 'B' represents a revealed blank square that has no adjacent (above, below, left, right, and all 4 diagonals) mines, digit ('1' to '8') represents how many mines are adjacent to this revealed square, and finally 'X' represents a revealed mine.

Now given the next click position (row and column indices) among all the unrevealed squares ('M' or 'E'), return the board after revealing this position according to the following rules:

If a mine ('M') is revealed, then the game is over - change it to 'X'.

If an empty square ('E') with no adjacent mines is revealed, then change it to revealed blank ('B') and all of its adjacent unrevealed squares should be revealed recursively.

If an empty square ('E') with at least one adjacent mine is revealed, then change it to a digit ('1' to '8') representing the number of adjacent mines.

Return the board when no more squares will be revealed.

Example 1:

Input:

[['E', 'E', 'E', 'E', 'E'],

['E', 'E', 'M', 'E', 'E'],

['E', 'E', 'E', 'E', 'E'],

['E', 'E', 'E', 'E', 'E']]

Click : [3,0]

Output:

[['B', '1', 'E', '1', 'B'],

['B', '1', 'M', '1', 'B'],

['B', '1', '1', '1', 'B'],

['B', 'B', 'B', 'B', 'B']]

Example 2:

Input:

[['B', '1', 'E', '1', 'B'],

['B', '1', 'M', '1', 'B'],

['B', '1', '1', '1', 'B'],

['B', 'B', 'B', 'B', 'B']]

Click : [1,2]

Output:

[['B', '1', 'E', '1', 'B'],

['B', '1', 'X', '1', 'B'],

['B', '1', '1', '1', 'B'],

['B', 'B', 'B', 'B', 'B']]

Note:

The range of the input matrix's height and width is [1,50].

The click position will only be an unrevealed square ('M' or 'E'), which also means the input board contains at least one clickable square.

The input board won't be a stage when game is over (some mines have been revealed).

For simplicity, not mentioned rules should be ignored in this problem. For example, you don't need to reveal all the unrevealed mines

when the game is over, consider any cases that you will win the game or flag any squares.

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.

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.

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.