expliquer moi ce code etapes par etapes : from queue import Queue

adj$\_$list $=\{$

$"$A$"$:$["$B$","$D$"],$

$"$B$"$:$["$A$","$C$"],$

$"$C$"$:$["$B$"],$

$"$D$"$:$["$A$","$E$","$F$"],$

$"$E$"$:$["$D$","$F$","$G$"],$

$"$F$"$:$["$D$","$E$","$H$"],$

$"$G$"$:$["$E$","$H$"],$

$"$H$"$:$["$G$","$F$"],$

$\}$

#bfs

visited $=\{\}$

level $=\{\}$

parent $=\{\}$

bfs$\_$traversal$\_$output $=[]$

queue $=$ Queue$()$

for node in adj$\_$list.keys$()$:

visited$[$node$]=$ False

parent$[$node$]=$ None

level$[$node$]=-1$

S$="$A$"$

visited$[$S$]=$ True

level$[$S$]=0$

queue.put$($S$)$

while not queue.empty$()$:

u $=$ queue.get$()$

bfs$\_$traversal$\_$output.append$($u$)$

for v in adj$\_$list$[$u$]$:

if not visited$[$v$]$:

visited$[$v$]=$ True

parent$[$v$]=$ u

level$[$v$]=$ level$[$u$]+1$

queue.put$($v$)$

print$($bfs$\_$traversal$\_$output$)$