# 481. Magical String

## Problem

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 s is 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 is s itself.

Given an integer n, return the number of 1’s in the first n number in the magical string s.

Example 1:

```
Input: n = 6
Output: 3
Explanation: The first 6 elements of magical string s is "122112" and it contains three 1's, so return 3.
```

Example 2:

```
Input: n = 1
Output: 1
```

Constraints:

- 1 <= n <= 10^5

## Code

```
class Solution {
public int magicalString(int n) {
if(n <= 3) return 1;
int[] arr = new int[n];
arr[0] = 1;
arr[1] = 2;
arr[2] = 2;
int countIndex = 2;
int endIndex = 3;
int currNum = 1;
int res = 1;
while(endIndex < n){
int currCount = arr[countIndex++];
int i = 0;
while(i < currCount && endIndex < n) {
arr[endIndex] = currNum;
endIndex++;
i++;
if(currNum == 1) res++;
}
currNum = currNum == 1 ? 2 : 1;
}
return res;
}
}
```

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