481. Magical String

https://leetcode.com/contest/leetcode-weekly-contest-14/problems/magical-string/

Solution:
Need to find some pattern for this question. After careful observation, it is found that:
1): the 1 and 2 groups appears alternatively;
2): the numbers of char inside each group are determined by the previous sequential char value in the same string (that's one of reasons why it is magical, as indicated);

Code:

public:
    int magicalString(int n) {
        if(n<1) return 0;
        if(n<4) return 1;
        int res = 1;
        string s = "122";
        for(int i=2, t=2; i<n; ++i){// t as the flag to be 1 or 2 for adding;
            if(s[i] == '1'){
                ++res;
                if(t == 1){
                    s += "2";
                    t = 2;
                }
                else{
                    s += "1";
                    t = 1;
                }
            }
            else{
                if(t == 1){
                    s += "22";
                    t = 2;
                }
                else{
                    s += "11";
                    t = 1;
                }
            }
        }
        return res;
    }
};