import check

def compound$\_$to$\_$list$($s: str$)->$ list$[$str$]$:

$"""$

Convert a chemical compound string into a list with each element repeated

the correct number of times as indicated by its subscript.

Parameters:

s $($str$)$: A string representing a chemical compound $($e$.$g$.,"$C$2$ H$5$ O H$")$

Returns:

list$[$str$]$: A list of elements, each repeated according to its subscript.

$"""$

elements $=[]$

i $=0$

while i $<$ len$($s$)$:

element $=$ s$[$i$]$

i $+=1$

if i $<$ len$($s$)$ and s$[$i$].$islower$()$:

element $+=$ s$[$i$]$

i $+=1$

count $=1$

if i $<$ len$($s$)$ and s$[$i$].$isdigit$()$:

num$\_$str $=""$

while i $<$ len$($s$)$ and s$[$i$].$isdigit$()$:

num$\_$str $+=$ s$[$i$]$

i $+=1$

count $=$ int$($num$\_$str$)$

elements.extend$([$element$]*$ count$)$

if i $<$ len$($s$)$ and s$[$i$]==\text{'}\text{'}$:

i $+=1$

return elements

check.expect$("$ethanol$",$ compound$\_$to$\_$list$("$C$2$ H$5$ O H$"),$

$["$C$","$C$","$H$","$H$","$H$","$H$","$H$","$O$","$H$"])$

check.expect$("$sodium phosphate", compound$\_$to$\_$list$("$Na$3$ P O$4"),$

$["$Na$","$Na$","$Na$","$P$","$O$","$O$","$O$","$O$"])$

def sort$\_$by$\_$num$\_$elements$($compounds: list$[$str$])->$ None:

$"""$

Mutate the given list of chemical compound strings so that they are sorted

by the number of elements they expand into, using the compound$\_$to$\_$list function.

Parameters:

compounds $($list$[$str$])$: A list of chemical compound strings.

Returns:

None: This function mutates the list in$-$place.

$"""$

compounds.sort$($key$=$lambda s: $($str$($compound$\_$to$\_$list$($s$)),$ s$))$

L $=["$Na$3$ P O$4","$C$2$ H$5$ O H$","$C$9"]$

check.expect$("$Sample of above", sort$\_$by$\_$num$\_$elements$($L$),$ None$)$

check.expect$("$Sample of above mutation", L$,[\text{'}$C$2$ H$5$ O H$\text{'},\text{'}$C$9\text{'},\text{'}$Na$3$ P O$4\text{'}])$