Leetcode #529: Minesweeper

func updateBoard(_ board: [[Character]], _ click: [Int]) -> [[Character]] {
    var board = board
    if board[click[0]][click[1]] == "M" {
        board[click[0]][click[1]] = "X"
        return board
    }
    helper(&board, click[0], click[1])
    return board
}

private func helper(_ board: inout [[Character]], _ i: Int, _ j: Int) {
    if i < 0 || i >= board.count || j < 0 || j >= board[i].count || board[i][j] != "E" {
        return
    }
    var count = 0
    if i-1 >= 0 && j-1 >= 0 && board[i-1][j-1] == "M" {
        count += 1
    }
    if i-1 >= 0 && board[i-1][j] == "M" {
        count += 1
    }
    if i-1 >= 0 && j+1 < board[i].count && board[i-1][j+1] == "M" {
        count += 1
    }
    if j-1 >= 0 && board[i][j-1] == "M" {
        count += 1
    }
    if j+1 < board[i].count && board[i][j+1] == "M" {
        count += 1
    }
    if i+1 < board.count && j-1 >= 0 && board[i+1][j-1] == "M" {
        count += 1
    }
    if i+1 < board.count && board[i+1][j] == "M" {
        count += 1
    }
    if i+1 < board.count && j+1 < board[i].count && board[i+1][j+1] == "M" {
        count += 1
    }
    if count == 0 {
        board[i][j] = "B"
        helper(&board, i-1, j-1)
        helper(&board, i-1, j)
        helper(&board, i-1, j+1)
        helper(&board, i, j-1)
        helper(&board, i, j+1)
        helper(&board, i+1, j-1)
        helper(&board, i+1, j)
        helper(&board, i+1, j+1)
    } else {
        board[i][j] = Character("\(count)")
    }
}