1 ^ 3 = 2

2 ^ 3 = 1

cpp
class Solution {
public:
    int magicalString(int n) {
        int res = 0;
        int tag = 2;
        string s1 = "";
        string s2 = "12";
        for (int i = 0; i < n; i++) {
            if (i >= s2.size()) s2 = s1;
            if (s2[i] == '1') res++;
            for (int j = 0; j < s2[i] - '0'; j++) {
                s1 += (tag^3) + '0';
            }
            tag ^= 3;
        }
        return res;
    }
};