leetcode481

public class Solution {
    public int MagicalString(int n) {
        if (n <= 0) return 0;
            if (n <= 3) return 1;

            int[] a = new int[n + 1];
            a[0] = 1; a[1] = 2; a[2] = 2;
            int head = 2, tail = 3, num = 1, result = 1;

            while (tail < n)
            {
                for (int i = 0; i < a[head]; i++)
                {
                    a[tail] = num;
                    if (num == 1 && tail < n)
                    {
                        result++;
                    }
                    tail++;
                }
                num = num ^ 3;//异或
                head++;
            }

            return result;
    }
}