## 481. 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:6Output:3Explanation:The first 6 elements of magical string S is "12211" and it contains three 1's, so return 3.

## Rust Solution

```
struct Solution;
impl Solution {
fn magical_string(n: i32) -> i32 {
if n == 0 {
return 0;
}
if n <= 3 {
return 1;
}
let n = n as usize;
let mut a = vec![1, 2, 2];
let mut i = 2;
let mut x = 1;
let mut res = 1;
loop {
for _ in 0..a[i] {
if x == 1 {
res += 1;
}
a.push(x);
if a.len() >= n {
return res;
}
}
x = 3 - x;
i += 1;
}
}
}
#[test]
fn test() {
let n = 6;
let res = 3;
assert_eq!(Solution::magical_string(n), res);
}
```

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