Palindromi

use std::{io::stdin};

fn main() {
    let stdin = stdin();
    let (n, k): (usize, usize) = {
        let mut s = String::new();
        stdin.read_line(&mut s).unwrap();
        let mut l = s.trim_end().split_whitespace();
        (l.next().unwrap().parse().unwrap(), l.next().unwrap().parse().unwrap())
    };

    let letters: Vec<char> = "abcdefghijklmnopqrstuvwxyz".chars().collect();

    let mut palindromi = String::with_capacity(n);

    for i in 0..(k/2) {
        palindromi.insert(i, letters[i % letters.len()]);
        palindromi.insert(palindromi.len() - 1 - i, letters[i % letters.len()]);
    }
    if k % 2 != 0 {
        palindromi.insert(k / 2, letters[(k / 2 + 1) % letters.len()]);
    }
    
    let mut offset: usize = if palindromi.len() > 0 { alphabet_index(palindromi.chars().last().unwrap()) + 1 } else { 0 };
    if palindromi.len() > 0 && n > k {
        let first_palindromi_char = palindromi.chars().next().unwrap();
        let first_palindromi_char_index: usize = alphabet_index(first_palindromi_char);
        if (n - k - 1 + offset) % letters.len() == first_palindromi_char_index {
            offset = 1;
        }
    }

    let mut fill = String::with_capacity(n - k);
    for i in 0..(n - k) {
        fill.push(letters[(i + offset) % letters.len()]);
    }

    assert_ne!(fill.chars().last(), palindromi.chars().next());
    assert_eq!(fill.len() + palindromi.len(), n);

    println!("{}", fill + &*palindromi)
}


// Debugger crashes for some reason with letters.iter().position(|c| c == first_palindromi_char).unwrap();
fn alphabet_index(character: char) -> usize {
    let letters: Vec<char> = "abcdefghijklmnopqrstuvwxyz".chars().collect();

    let mut index = 0;
    for (i, c) in letters.iter().enumerate() {
        if *c == character {
            index = i;
            break;
        }
    }
    index
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED