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