33. (481) Magical String - Leetcode.cpp

/*
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";
        for (int i = 3; i <= n; ++i) 
            s.append(s[i] - '0', i & 1 ? '1' : '2');
        return count(s.begin(), s.begin() + n + 1, '1');
    }
};