# LeetCode 481  Magical String

## 题目描述

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 Sitself.

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.
```

## 一句话题意

*如果将字符串按照1和2分组后，统计每组1或2出现的次数所拼接成的字符串，和这个字符串本身一样

## 解题思路

```class Solution {
public:
int magicalString(int n) {
string s = "122";
int i = 2;
while (s.size() < n) {
s += string(s[i++] - '0', s.back() ^ 3);
}
return count(s.begin(), s.begin() + n, '1');
}
};```