Problem:
Score : 300 points

Problem Statement

You are given a string s.
Among the different substrings of s, print the K-th lexicographically smallest one.

A substring of s is a string obtained by taking out a non-empty contiguous part in s.
For example, if s = ababc, a, bab and ababc are substrings of s, while ac, z and an empty string are not.
Also, we say that substrings are different when they are different as strings.

Let X = x_{1}x_{2}...x_{n} and Y = y_{1}y_{2}...y_{m} be two distinct strings. X is lexicographically larger than Y if and only if Y is a prefix of X or x_{j} > y_{j} where j is the smallest integer such that x_{j} \neq y_{j}.

Constraints

1 ≤ |s| ≤ 5000

s consists of lowercase English letters.

1 ≤ K ≤ 5

s has at least K different substrings.

Partial Score

200 points will be awarded as a partial score for passing the test set satisfying |s| ≤ 50.

Input

Input is given from Standard Input in the following format:

s
K

Output

Print the K-th lexicographically smallest substring of K.

Sample Input 1

aba
4

Sample Output 1

b

s has five substrings: a, b, ab, ba and aba.
Among them, we should print the fourth smallest one, b.
Note that we do not count a twice.

Sample Input 2

atcoderandatcodeer
5

Sample Output 2

andat

Sample Input 3

z
1

Sample Output 3

z

Solution in Rust (1.42.0) (judge verdict: Accepted)

#![allow(non_snake_case)]
#![allow(unused_imports)]

use proconio::{input, fastout};
use proconio::marker::*;
use whiteread::parse_line;
use std::collections::*;
use num::*;
use num_traits::*;
use superslice::*;
use std::ops::*;

use itertools::Itertools;
use itertools_num::ItertoolsNum;

fn solve() {
    input!{
        s: Chars,
        K: usize,
    }

    let mut length = if s.len() < 5 { s.len() } else { 5 };
    let mut bset_all: BTreeSet<String> = BTreeSet::new();

    while length > 0 {
        // dbg!(&bset_all);
        let mut tmp_bset: BTreeSet<String> = BTreeSet::new();
        for i in 0..(s.len()-length+1) {
            let tmp_string = s[i..i+length].iter().map(|c| *c).collect::<String>();
            if !bset_all.contains(&tmp_string) {
                tmp_bset.insert(tmp_string);
            }
        }

        bset_all.append(&mut tmp_bset);
        length -= 1;
    }

    for (i, str) in bset_all.iter().enumerate() {
        if i == K-1 { println!("{}", str) }
    }
}

fn main() {
    solve()
}

#[cfg(test)]
mod test {
    use super::solve;
}