> For the complete documentation index, see [llms.txt](https://ugopireddy.gitbook.io/leet-code-solutions/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ugopireddy.gitbook.io/leet-code-solutions/solutions-451-500/481-magical-string.md).

# 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 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.
```

### Solutions:

```java
public class Solution {
    public int magicalString(int n) {
        if (n <= 0) {
            return 0;
        }
        int[] data = new int[100002];
        data[0] = 1;
        int i = 0, j = 0;
        int count = 1;
        while (i < n) {
            if (data[j] == 1) {
                if (data[i] == 1) {
                    data[++i] = 2;
                }
                else {
                    data[++i] = 1;
                    if (i < n) {
                        count ++;
                    }
                }
            }
            else {
                if (data[i] == 1) {
                    data[++i] = 1;
                    if (i < n) {
                        count ++;
                    }
                    data[++i] = 2;
                }
                else {
                    data[++i] = 2;
                    data[++i] = 1;
                    if (i < n) {
                        count ++;
                    }
                }
            }
            j ++;
        }
        return count;
    }
}
```

```java
public class Solution {
    public int magicalString(int n) {
        if (n <= 0) {
            return 0;
        }
        if (n <= 3) {
            return 1;
        }
        int[] data = new int[100002];
        data[0] = 1;
        data[1] = 2;
        data[2] = 2;
        
        int i = 2, j = 2;
        int count = 1;
        int num = 1;
        while (i < n) {
            for (int k = 0; k < data[j]; k ++) {
                data[++i] = num;
                if (num == 1 && i < n) {
                    count ++;
                }
            }
            j ++;
            num = num ^ 3;
        }
        return count;
    }
}
```

```java
public class Solution {
    public int magicalString(int n) {
        if (n <= 0) {
            return 0;
        }
        if (n <= 3) {
            return 1;
        }
        StringBuilder sb = new StringBuilder("122");
        int i = 2;
        while (sb.length() < n) {
            int num = (sb.charAt(sb.length() - 1) - '0') ^ 3;
            int repeat = sb.charAt(i) - '0';
            for (int j = 0; j < repeat; j ++) {
                sb.append(num);
            }
            i ++;
        }
        int count = 0;
        for (i = 0; i < n; i ++) {
            if (sb.charAt(i) == '1') {
                count ++;
            }
        }
        return count;
    }
}
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://ugopireddy.gitbook.io/leet-code-solutions/solutions-451-500/481-magical-string.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.