leetcode 481. Magical String python

A magical string S consists of only '1' and '2' and obeys the following rules:
The string S is magical because concatenating the number of contiguous occurrences of characters '1' and '2' generates the string
S itself.

The first few elements of string S is the following:
S = "1221121221221121122……"

If we group the consecutive '1's and '2's in S, it will be:

1 22 11 2 1 22 1 22 11 2 11 22 ......

and the occurrences of '1's or '2's in each group are:

1 2 2 1 1 2 1 2 2 1 2 2 ......

You can see that the occurrence sequence above is the S itself.

Given an integer N as input, return the number of '1's in the first N number in the magical string
S.

Note: N will not exceed 100,000.

Example 1:

Input: 6Output: 3Explanation: The first 6 elements of magical string S is "12211" and it contains three 1's, so return 3.

class Solution(object):
def magicalString(self, n):
"""
:type n: int
:rtype: int
"""
S = [1,2,2]
idx = 2
while len(S) < n:
S += S[idx] * [(3 - S[-1])]
idx += 1
return S[:n].count(1)