Nice one

MMMAAANNN's two line solution does l=248 in 1 millisecond but any higher causes maximum recursion error.

My point is that this solution is O(2^n) too, so your point is invalid.

That is exactly my point initially, not sure why are you arguing.

That's because the random tests are woefully weak: only 20 chars max.

Why not try it yourself? ;-) Like this:

l = 50
p1 = 'a'*l+'b'
p2 = 'a'*l+'c'

print(is_merge(p1+p2, p1, p2))
print(is_merge(p2+p1, p1, p2))
# etc etc...

This test obliterates most existing solution easily. If not, increase l to 100 or something.

And your solution fails on this case:

is_merge('aaaabc','aab','aac') # should be True

Python can crush with an error using recursion on bigger values. This one would at least work.
My solution has linear time and constant memory complexity (although it may not look pretty as some one-liners on this site). I think it's the best we can do here.

Nope, it's still the same as the recursive ones. It'll still timeout badly for long strings.

Nope, it's still the same as the recursive ones. It'll still timeout badly for long strings.

This should be voted as Best Practice!

I think this solution is better then the ones with all of the recursive functions.

So nice.