$[]$ # Tests $10$ points: More tests for is$\_$tree

$g=$ Graph$()$

assert g$.$is$\_$tree

Exercise: Write a function number$\_$of$\_$islands that counts the number of islands of horizonally$-$or$-$vertically connected $1$ s in a matrix of $0,1$ elements.

Exercise: Write a has$\_$loop method that checks whether a graph has a loop.