Magical string

Two Pointers String

54.6% acceptance

Jan 13, 2026

376

1419

A magical string s consists of only '1' and '2' and obeys the following rule: Concatenating the sequence of lengths of its consecutive groups of identical 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 counting the occurrences of 1's or 2's in each group yields the sequence "1 2 2 1 1 2 1 2 2 1 2 2 ......". You can see that concatenating the occurrence sequence gives us s itself. Given an integer n, return the number of 1's in the first n number in the magical string s.

Solution

Rust

Time O(n²)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn magical_string(n: i32) -> i32 {
    if n == 0 { return 0; }
    if n <= 3 { return 1; }
    
    let n = n as usize;
    let mut s = vec![1, 2, 2];
    let mut head = 2;
    let mut tail = 3;
    let mut num = 1;
    
    while tail < n {
      for _ in 0..s[head] {
        if tail < n {
          s.push(num);
          tail += 1;
        }
      }
      num = 3 - num;
      head += 1;
    }
    
    s.iter().take(n).filter(|&&x| x == 1).count() as i32
  }
}