func numIslands(_ grids: [[Character]]) -> Int {
    if grids.isEmpty || grids[0].isEmpty {
        return 0
    }
    var count = 0
    var grids = grids
    for i in 0..<grids.count {
        for j in 0..<grids[0].count {
            if grids[i][j] == "1" {
                count += 1
                helper(&grids, i, j)
            }
        }
    }
    return count
}

private func helper(_ grids: inout [[Character]], _ x: Int, _ y: Int) {
    guard x >= 0 && x < grids.count && y >= 0 && y < grids[0].count else {
        return
    }
    if grids[x][y] == "1" {
        grids[x][y] = "X"
        helper(&grids, x-1, y)
        helper(&grids, x+1, y)
        helper(&grids, x, y-1)
        helper(&grids, x, y+1)
    }
}