Understood. I should have read the question better. Thanks for the help, it worked.

My code works with the test codes, but fails at submission on this inputs:

  a1 = ['1295', 'code', '1346', '1028', 'ar']
  a2 = ['12951295', 'ode', '46', '10281066', 'par']

My code returns: ['1028', '1295', '46', 'ar', 'ode']
But it says it should return: ['1028', '1295', 'ar']

I don't get it.. it should catch substrings as well right?? Why should it return just should equal ['1028', '1295', 'ar']?

Never mind! I guess sum(range(int)) is not as efficient.
For anyone facing this issue, you should figure out another way to do it.
Hint:A guy called Gauss solved this a while ago :)

use integer instead. Something like:
m = int(m)
n = int(n)

use integer instead. Something like:
m = int(m)
n = int(n)

I think this solution doesn't insert a node in the first position (0).

I think this solution doesn't insert a node in the first position (0).

I think this solution doesn't insert a node in the first position (0).

I think this solution doesn't insert a node in the first position (0).

Thank you guys. I was returning the data value instead of the node object.