I think string is: result = str(b) +result
During this operation:
- new string with size  = result.size+1 created;
- b contents copied to new string
I am not sure about memory but it must lead to TLE.

You know your n1, n2, result strings have known size = n.
You should rather use efficient byte arrays of size n to store n1, n2, result contents

Edited by author 28.04.2016 13:22

if You get TLE verdict, U can try to use short instead of int