In Python 3, str is unicode strings. They take more space than byte strings. This may be one of the reasons for getting Runtime Error.

_{Code Python 2}

>>> import sys
>>> print(sum([sys.getsizeof(i) for i in '12345689'*10**5]))
17600000  

_{Code Python 3}

>>> import sys
>>> print(sum([sys.getsizeof(i) for i in '12345689'*10**5]))
20800000

To overcome this, we can simply avoid creating a huge list, which is causing memory error.

I have added a solution in Python 3 in Editorial.

^{_{*One can also, try solving this using Counters.}}