## 题目描述：

LeetCode 481. Magical String

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

## 题目大意：

S的起始部分元素为：S = "1221121221221121122……"

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

`1 2    2 1 1 2 1 2 2 1 2 2 ......`

## Python代码：

``````class Solution(object):
def magicalString(self, n):
"""
:type n: int
:rtype: int
"""
ms = '122'
p = 2
while len(ms) < n:
ms += str(3 - int(ms[-1])) * int(ms[p])
p += 1
return ms[:n].count('1')
``````

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