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: 6
Output: 3
Explanation: 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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