func dfs(grid [][]byte, i, j int) {
if i < 0 || i >= len(grid) || j < 0 || j >= len(grid[0]) || grid[i][j] != '1' {
return
}
grid[i][j] = '2'
dfs(grid, i-1, j)
dfs(grid, i, j-1)
dfs(grid, i+1, j)
dfs(grid, i, j+1)
}
func numIslands(grid [][]byte) int {
res := 0
for i := range grid {
for j := range grid[0] {
if grid[i][j] == '1' {
dfs(grid, i, j)
res++
}
}
}
return res
}