481. 神奇字符串

 1 //1与2交替出现，从下标2开始
 2 class Solution 
 3 {
 4 public:
 5     int magicalString(int n) 
 6     {
 7         string s = "122";
 8         for(int i = 2,k = 1;i < n;i ++,k = 3 - k)
 9         {
10             for(int j = 0;j < s[i] - '0';j ++) s += to_string(k);
11         }
12         int res = 0;
13         for(int i = 0;i < n;i ++) res += s[i] == '1';
14         return res;
15     }
16 };