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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
	"sort"
	"strings"
)

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	var s string
	scanner.Scan()
	fmt.Sscanf(scanner.Text(), "%s", &s)
	chars := strings.Split(s, "")

	var K int
	scanner.Scan()
	fmt.Sscanf(scanner.Text(), "%d", &K)

	set := NewStrSet()
	for i := 0; i < len(chars); i++ {
		for k := 1; k <= 5 && i+k <= len(chars); k++ {
			str := strings.Join(chars[i:i+k], "")
			set.Add(str)
		}
	}
	//fmt.Println(set.Entries())

	records = make([]Record, set.Size())
	index := 0
	for str, _ := range set.Entries() {
		records[index] = Record{str}
		index++
	}
	sort.Sort(records)
	//fmt.Println(records)
	fmt.Println(records[K-1].str)

}

var records Records

type Record struct {
	str string
}

type Records []Record

func (r Records) Len() int {
	return len(r)
}
func (r Records) Swap(i, j int) {
	r[i], r[j] = r[j], r[i]
}

func (r Records) Less(i, j int) bool {
	return r[i].str < r[j].str
}

type StringSet struct {
	set map[string]struct{}
}

func NewStrSet() *StringSet {
	return &StringSet{
		set: make(map[string]struct{}),
	}
}

func (s *StringSet) Add(str string) {
	s.set[str] = struct{}{}
}

func (s *StringSet) Contains(str string) bool {
	_, e := s.set[str]
	return e
}

func (s *StringSet) Size() int {
	return len(s.set)
}

func (s *StringSet) IsEmpty() bool {
	return s.Size() == 0
}

func (s *StringSet) Entries() map[string]struct{} {
	return s.set
}