Concat (maximize) の解説

fn main() {
    input! {
        n: usize, k: usize,
        mut s: [String; n]
    }
    s.sort_by(|a, b| {
        let aln = a.len();
        let bln = b.len();
        if aln != bln {
            return bln.cmp(&aln);
        }
        b.cmp(&a)
    });
    let f = |mut s: Vec<String>| {
        s.sort_by(|a, b| {
            let aa = format!("{}{}", a, b);
            let bb = format!("{}{}", b, a);
            bb.cmp(&aa)
        });
        s.concat()
    };
    let ans1 = f(s[..k].to_owned());
    let mut best = s[k - 1].parse::<usize>().unwrap();
    for i in k..n {
        let num = s[i].parse::<usize>().unwrap();
        best = max(best, num);
    }
    let mut cur = s[..k].to_vec();
    cur.pop().unwrap();
    cur.push(best.to_string());
    let ans2 = f(cur);

    let mut ans1 = ans1.trim_start_matches("0");
    let mut ans2 = ans2.trim_start_matches("0");

    if ans1.is_empty() {
        ans1 = "0";
    }
    if ans2.is_empty() {
        ans2 = "0";
    }

    let ord = 'l: {
        if ans1.len() != ans2.len() {
            break 'l ans2.len().cmp(&ans1.len());
        }
        ans2.cmp(&ans1)
    };
    let ans = if ord == Ordering::Greater {ans2} else {ans1};
    println!("{}", ans);
}