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 s is 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 is s itself.

Given an integer n, return the number of 1’s in the first n number in the magical string s.

class Solution {
public:
    int magicalString(int n) {
        string s = "122";
        int p = 2;
        while (s.size() < n) s += string(s[p++] & 0b1111, s.back()^3);
        return count(s.begin(), s.begin() + n, '1');
    }
};