Problem p03353 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <bitset>
#include <cmath>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <string>
#include <tuple>
#include <vector>

template <typename T> bool chmax(T &a, const T &b) {
  if (a <= b) {
    a = b;
    return (true);
  } else {
    return (false);
  }
}
template <typename T> bool chmin(T &a, const T &b) {
  if (a >= b) {
    a = b;
    return (true);
  } else {
    return (false);
  }
}

using namespace std;

using ll = long long;
using ull = unsigned long long;

using Pll = pair<ll, ll>;
using Pull = pair<ull, ull>;

#define eb emplace_back
#define pb push_back
#define mp make_pair
#define mt make_tuple

#define F first
#define S second

#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define reps(i, n) for (int i = 1; i <= (int)(n); ++i)
#define rrep(i, n) for (int i = (int)((n)-1); i >= 0; --i)
#define rreps(i, n) for (int i = (int)((n)); i > 0; --i)
#define arep(i, v) for (auto &&i : (v))

template <typename T> T gcd(const T a, const T b) {
  return (b ? gcd(b, a % b) : a);
}

#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define UNIQUE(c) (c).erase(unique((c).begin(), (c).end()), (c).end())

constexpr ll MOD = 1000000007LL;

#define y0 y3487465
#define y1 y8687969
#define j0 j1347829
#define j1 j234892
#define next asdnext
#define prev asdprev

template <typename T = ll> class UnionFind {
public:
  UnionFind(T n) {
    rep(i, n) {
      par[i] = i;
      siz[i] = 1;
    }
  }
  T find(T x) {
    if (x == par[x])
      return (x);
    else
      return (par[x] = find(par[x]));
  }
  void unite(T x, T y) {
    T xx = find(x);
    T yy = find(y);
    if (xx == yy)
      return;
    if (siz[xx] <= siz[yy])
      swap(xx, yy);
    par[yy] = xx;
    siz[xx] += siz[yy];
  }

private:
  vector<T> par, siz;
};

template <typename T = ll> T power(T a, T b, T m = MOD) {
  T res = 1;
  while (b > 0) {
    if (b & 1)
      res = res * a % m;
    a = a * a % m;
    b >>= 1;
  }
  return (res);
}

/*
constexpr ll MAX = 500010;
ll fact[MAX];
ll inv[MAX];
ll inv_fact[MAX];

template<typename T> void initComb( T n, T m = MOD )
{
        fact[0] = fact[1] = inv_fact[0] = inv_fact[1] = 1;
        inv[1] = 1;
        for ( int i = 2; i < n; i++ ) {
                fact[i] = ( fact[i - 1] * i ) % m;
                inv[i] = m - inv[m % i] * ( m / i ) % m;
                inv_fact[i] = inv_fact[i - 1] * inv[i] % m;
        }
}

template<typename T> T comb( T n, T r, T m = MOD )
{
        if ( n < r ) return ( 0 );
        if ( n < 0 || r < 0 ) return ( 0 );
        return ( fact[n] * ( inv_fact[r] * inv_fact[n - r] % m ) % m );
}
*/

void replace(string &s, string t, string r) {
  string::size_type p = 0;
  while ((p = s.find(t, p)) != string::npos) {
    s.replace(p, t.length(), r);
    p += r.length();
  }
}

int main() {
  string s;
  int K;
  cin >> s >> K;

  priority_queue<string, vector<string>, greater<string>> q;
  map<string, int> m;

  int N = s.size();
  rep(i, N) {
    for (int j = 1; j <= N - i; j++) {
      m[s.substr(i, j)] = 1;
    }
  }

  arep(it, m) { q.emplace(it.F); }

  rep(i, K - 1) { q.pop(); }

  cout << q.top() << endl;

  return (0);
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cmath>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <string>
#include <tuple>
#include <vector>

template <typename T> bool chmax(T &a, const T &b) {
  if (a <= b) {
    a = b;
    return (true);
  } else {
    return (false);
  }
}
template <typename T> bool chmin(T &a, const T &b) {
  if (a >= b) {
    a = b;
    return (true);
  } else {
    return (false);
  }
}

using namespace std;

using ll = long long;
using ull = unsigned long long;

using Pll = pair<ll, ll>;
using Pull = pair<ull, ull>;

#define eb emplace_back
#define pb push_back
#define mp make_pair
#define mt make_tuple

#define F first
#define S second

#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define reps(i, n) for (int i = 1; i <= (int)(n); ++i)
#define rrep(i, n) for (int i = (int)((n)-1); i >= 0; --i)
#define rreps(i, n) for (int i = (int)((n)); i > 0; --i)
#define arep(i, v) for (auto &&i : (v))

template <typename T> T gcd(const T a, const T b) {
  return (b ? gcd(b, a % b) : a);
}

#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define UNIQUE(c) (c).erase(unique((c).begin(), (c).end()), (c).end())

constexpr ll MOD = 1000000007LL;

#define y0 y3487465
#define y1 y8687969
#define j0 j1347829
#define j1 j234892
#define next asdnext
#define prev asdprev

template <typename T = ll> class UnionFind {
public:
  UnionFind(T n) {
    rep(i, n) {
      par[i] = i;
      siz[i] = 1;
    }
  }
  T find(T x) {
    if (x == par[x])
      return (x);
    else
      return (par[x] = find(par[x]));
  }
  void unite(T x, T y) {
    T xx = find(x);
    T yy = find(y);
    if (xx == yy)
      return;
    if (siz[xx] <= siz[yy])
      swap(xx, yy);
    par[yy] = xx;
    siz[xx] += siz[yy];
  }

private:
  vector<T> par, siz;
};

template <typename T = ll> T power(T a, T b, T m = MOD) {
  T res = 1;
  while (b > 0) {
    if (b & 1)
      res = res * a % m;
    a = a * a % m;
    b >>= 1;
  }
  return (res);
}

/*
constexpr ll MAX = 500010;
ll fact[MAX];
ll inv[MAX];
ll inv_fact[MAX];

template<typename T> void initComb( T n, T m = MOD )
{
        fact[0] = fact[1] = inv_fact[0] = inv_fact[1] = 1;
        inv[1] = 1;
        for ( int i = 2; i < n; i++ ) {
                fact[i] = ( fact[i - 1] * i ) % m;
                inv[i] = m - inv[m % i] * ( m / i ) % m;
                inv_fact[i] = inv_fact[i - 1] * inv[i] % m;
        }
}

template<typename T> T comb( T n, T r, T m = MOD )
{
        if ( n < r ) return ( 0 );
        if ( n < 0 || r < 0 ) return ( 0 );
        return ( fact[n] * ( inv_fact[r] * inv_fact[n - r] % m ) % m );
}
*/

void replace(string &s, string t, string r) {
  string::size_type p = 0;
  while ((p = s.find(t, p)) != string::npos) {
    s.replace(p, t.length(), r);
    p += r.length();
  }
}

int main() {
  string s;
  int K;
  cin >> s >> K;

  priority_queue<string, vector<string>, greater<string>> q;
  map<string, int> m;

  int N = s.size();
  rep(i, N) {
    for (int j = 1; j <= K; j++) {
      m[s.substr(i, j)] = 1;
    }
  }

  arep(it, m) { q.emplace(it.F); }

  rep(i, K - 1) { q.pop(); }

  cout << q.top() << endl;

  return (0);
}