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 string **S** is the following: **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 above is the **S** itself.

Given an integer N as input, return the number of '1's in the first N number in the magical string **S**.

Input:6Output:3Explanation:The first 6 elements of magical string S is "12211" and it contains three 1's, so return 3.

Solution:

class Solution { public int magicalString(int n) { if (n == 0) return 0; if (n <= 3) return 1; int[] arr = new int[n]; arr[0] = 1; arr[1] = 2; arr[2] = 2; int head = 2, tail = 3, num = 1, result = 1; while (tail < n) { for (int i = 0; i < arr[head] && tail < n; i ++) { arr[tail] = num; if (num == 1) result ++; tail ++; } head ++; num = 3 - num; } return result; } }