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.Subscribe to see which companies asked this question.
计算题目中描述的有规律的字符串的前n个字符中‘1’的个数。先构造这个字符串（大小大于n即可），然后计算其中‘1’的个数。规律是当前i指向的数表示次数，当前字符串的末尾的数的“相反数”表示要添加的数，比如“122”，i=2指向2，字符串末尾为“2”，即在字符串后加2个1.
代码：
class Solution { public: int magicalString(int n) { string s = "122"; int i = 2; while(s.size() < n) { s += string(s[i++] - '0', s.back() == '1' ? '2' : '1'); } return count(s.begin(), s.begin() + n, '1'); } };
