f$29.$ Using Big$-$O notation, what is the worst$-$case running time of the following

public static string f$29($int N$)\{$

if $($N $<1)\{$

return $""$;

$\}$

ArrayList result $=$ new ArrayList$()$:

result.add$("1")$;

for $($int i $=0$; i $<$ N; i$++)\{$

int size $=$ result.size$()$;

for $($int j $=$ size $/2$; j $<$ size; j$++)\{$

String aset $=$ result.get$($j$)$;

result.add$($ aset $+"0")$;

result.add$($ aset $+"1")$;

$\}$

$\}$

return result;

$\}$