D.cpp

// #pragma GCC optimize("Ofast,unroll-loops")
// #pragma GCC target("avx,avx2,fma")

#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define ull unsigned long long
#define dd double
#define ld long double
#define sl(n) scanf("%lld", &n)
#define si(n) scanf("%d", &n)
#define sd(n) scanf("%lf", &n)
#define pll pair <ll, ll>
#define pii pair <int, int>
#define mp make_pair
#define pb push_back
#define all(v) v.begin(), v.end()
#define inf (1LL << 61)
#define loop(i, start, stop, inc) for(ll i = start; i <= stop; i += inc)
#define for1(i, stop) for(ll i = 1; i <= stop; ++i)
#define for0(i, stop) for(ll i = 0; i < stop; ++i)
#define rep1(i, start) for(ll i = start; i >= 1; --i)
#define rep0(i, start) for(ll i = (start-1); i >= 0; --i)
#define ms(n, i) memset(n, i, sizeof(n))
#define casep(n) printf("Case %lld:", ++n)
#define pn printf("\n")
#define pf printf
#define EL '\n'
#define fastio std::ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);

// === DEBUG MACRO STARTS HERE === //
#ifndef ONLINE_JUDGE
#define DEBUG
#define SYS_COL system("COLOR")
#endif

int recur_depth = 0;
#ifdef DEBUG
#define dbg(x) {++recur_depth; auto x_=x; --recur_depth; SYS_COL; \
                cerr<<string(recur_depth, '\t')<<"\e[91m"<<__func__<<":" \
                <<__LINE__<<"\t"<<#x<<" = "<<x_<<"\e[39m"<<endl;}

template<typename Ostream, typename Cont>
typename enable_if<is_same<Ostream,ostream>::value,
            Ostream&>::type operator<<(Ostream& os,  const Cont& v) {
	os<<"[";
	for(auto& x:v){os<<x<<", ";}
	return os<<"]";
}
template<typename Ostream, typename ...Ts>
Ostream& operator<<(Ostream& os,  const pair<Ts...>& p){
	return os<<"{"<<p.first<<", "<<p.second<<"}";
}
#else
#define dbg(x)
#endif
// === DEBUG MACRO ENDS HERE === //

#define ff first
#define ss second

vector <pii> lst;
vector <int> pos[26];
string s;
ll k;

void solve(ll l, ll r)
{
    if(lst.size() >= k || l>r) return;

    for0(i, 26) {
        char ch = 'a'+i;
        ll l2 = lst.size();

        for(ll j = l; j <= r && lst.size() < k; j++) {
            ll x = lst[j].ss;
            if(x+1 < s.size() && s[x+1] == ch)
                lst.pb(mp(lst[j].ff, x+1));
        }

        solve(l2, (ll)lst.size()-1);
    }
}

int main()
{
    fastio;
    cin >> s >> k;

    for(ll i = 0; i < s.size(); i++) {
        ll now = s[i] - 'a';
        pos[now].pb(i);
    }

    for0(i, 26) {
        int l = lst.size();

        for(int &x : pos[i]) {
            if(lst.size()==k) break;

            lst.pb(mp(x, x));
        }

        solve(l, (ll)lst.size()-1);
    }

    if(lst.size() < k)
        cout << "No such line." << EL;
    else {
        for(ll i = lst.back().ff; i <= lst.back().ss; i++)
            cout << s[i];
        cout << EL;
    }

    return 0;
}