leetcode481. Magical String

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.

Note: N will not exceed 100,000.

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

    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 leftPointer = 2, rightPointer = 3, num = 1, count = 1;
        while(rightPointer < n) {
            for(int i = 0 ; i < a[leftPointer] && rightPointer < n ; i++) {
                a[rightPointer++] = num;
                if(num == 1) {
                    count++;
                }
            }
            leftPointer++;
            num ^= 3;
        }
        return count;
    }