Problem p03629 (C++)

Buggy submission (status: Runtime Error):
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
// #define int long long
// #define double long double
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define rep1(i, n) for (int i = 1; i < (int)(n); ++i)
#define repeq(i, n) for (int i = 0; i <= (int)(n); ++i)
#define rep1eq(i, n) for (int i = 1; i <= (int)(n); ++i)
#define rrep(i, n) for (int i = (int)(n)-1; i >= 0; --i)
#define rrep1(i, n) for (int i = (int)(n)-1; i > 0; --i)
#define rrepeq(i, n) for (int i = (int)(n); i >= 0; --i)
#define rrep1eq(i, n) for (int i = (int)(n); i > 0; --i)
#define REP(i, a, b) for (int i = (int)(a); i <= (int)(b); ++i)
#define RREP(i, a, b) for (int i = (int)(a); i >= (int)(b); --i)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using vb = vector<bool>;
template <typename T> using Graph = vector<vector<T>>;
template <typename T> using Spacial = vector<vector<vector<T>>>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int MOD = 1e9 + 7;
const int MOD2 = 998244353;
// const double EPS = 1e-9;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
string interval[2] = {" ", "\n"}; // {" ", "\n"}

template <typename T> struct is_plural : false_type {};
template <typename T1, typename T2>
struct is_plural<pair<T1, T2>> : true_type {};
template <typename T> struct is_plural<vector<T>> : true_type {};
template <typename T> struct is_plural<complex<T>> : true_type {};

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  return is >> p.first >> p.second;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  return os << p.first << " " << p.second;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (auto itr = vec.begin(); itr != vec.end(); ++itr)
    is >> *itr;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  if (vec.empty())
    return os;
  bool pl = is_plural<T>();
  os << vec.front();
  for (auto itr = ++vec.begin(); itr != vec.end(); ++itr)
    os << interval[pl] << *itr;
  return os;
}

bool CoutYN(bool a, string y = "Yes", string n = "No") {
  cout << (a ? y : n) << "\n";
  return a;
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

long long modpow(int a, long long n, int mod = MOD) {
  long long ret = 1;
  do {
    if (n & 1)
      ret = ret * a % mod;
    a = 1LL * a * a % mod;
  } while (n >>= 1);
  return ret;
}

template <typename T> T GCD(T a, T b) { return b ? GCD(b, a % b) : a; }
template <typename T> T LCM(T a, T b) { return a / GCD(a, b) * b; }

template <typename T1, typename T2>
bool CompareBySecond(pair<T1, T2> a, pair<T1, T2> b) {
  return a.second != b.second ? a.second < b.second : a.first < b.first;
}
// counterclockwise from 12 o'clock direction
template <typename T> bool CompareBySlope(pair<T, T> a, pair<T, T> b) {
  if (a.first * b.first < 0)
    return a.first < b.first;
  if (a.first == 0)
    return a.second >= 0 || b.first > 0;
  if (b.first == 0)
    return b.second < 0 && a.first < 0;
  return a.second * b.first < a.first * b.second;
}

/* -------- <templates end> -------- */

struct path {
  int sz, to;
  char c;

  path(int sz, int to, char c = 0) : sz(sz), to(to), c(c) {}
};

path zero(3e5, -1);

void calc(Graph<int> &pos, vector<path> &dp, int pre, int i, char c) {
  rep(j, 26) {
    auto itr = lower_bound(ALL(pos[j]), pre);

    if (*itr == -1) {
      if (zero.sz < dp[i].sz + 1)
        continue;
      if (zero.sz == dp[i].sz + 1 && zero.c < c)
        continue;

      zero = {dp[i].sz + 1, i, c};
    }

    else if (pre <= *itr) {
      if (dp[*itr].sz < dp[i].sz + 1)
        continue;
      if (dp[*itr].sz == dp[i].sz + 1 && dp[*itr].c < c)
        continue;

      dp[*itr] = {dp[i].sz + 1, i, c};
    }
  }
}

void solve() {
  string s;
  cin >> s;

  Graph<int> pos(26, vi(1, -1));
  int sz = s.size();
  rep(i, sz) { pos[s[i] - 'a'].emplace_back(i); }

  vector<path> dp(sz + 1, path(3e5, sz));
  dp[sz] = {0, -1};

  rep(i, 26) {
    if (pos[i].back() == -1) {
      cout << char('a' + i) << endl;
      return;
    }

    dp[sz].c = 'a' + i;
    calc(pos, dp, pos[i].back(), sz, dp[sz].c);
  }

  zero.to = sz;

  rrep(i, sz) {
    auto pre = lower_bound(ALL(pos[s[i] - 'a']), i);
    --pre;
    calc(pos, dp, *pre, i, s[i]);
  }

  int idx = zero.to;
  string ans;
  ans.push_back(zero.c);

  while (idx != sz) {
    ans.push_back(dp[idx].c);
    idx = dp[idx].to;
  }

  cout << ans << endl;
}

/* -------- <programs end> -------- */

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  solve();
  return 0;
}


Fixed submission:
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
// #define int long long
// #define double long double
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define rep1(i, n) for (int i = 1; i < (int)(n); ++i)
#define repeq(i, n) for (int i = 0; i <= (int)(n); ++i)
#define rep1eq(i, n) for (int i = 1; i <= (int)(n); ++i)
#define rrep(i, n) for (int i = (int)(n)-1; i >= 0; --i)
#define rrep1(i, n) for (int i = (int)(n)-1; i > 0; --i)
#define rrepeq(i, n) for (int i = (int)(n); i >= 0; --i)
#define rrep1eq(i, n) for (int i = (int)(n); i > 0; --i)
#define REP(i, a, b) for (int i = (int)(a); i <= (int)(b); ++i)
#define RREP(i, a, b) for (int i = (int)(a); i >= (int)(b); --i)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using vb = vector<bool>;
template <typename T> using Graph = vector<vector<T>>;
template <typename T> using Spacial = vector<vector<vector<T>>>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int MOD = 1e9 + 7;
const int MOD2 = 998244353;
// const double EPS = 1e-9;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
string interval[2] = {" ", "\n"}; // {" ", "\n"}

template <typename T> struct is_plural : false_type {};
template <typename T1, typename T2>
struct is_plural<pair<T1, T2>> : true_type {};
template <typename T> struct is_plural<vector<T>> : true_type {};
template <typename T> struct is_plural<complex<T>> : true_type {};

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  return is >> p.first >> p.second;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  return os << p.first << " " << p.second;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (auto itr = vec.begin(); itr != vec.end(); ++itr)
    is >> *itr;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  if (vec.empty())
    return os;
  bool pl = is_plural<T>();
  os << vec.front();
  for (auto itr = ++vec.begin(); itr != vec.end(); ++itr)
    os << interval[pl] << *itr;
  return os;
}

bool CoutYN(bool a, string y = "Yes", string n = "No") {
  cout << (a ? y : n) << "\n";
  return a;
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

long long modpow(int a, long long n, int mod = MOD) {
  long long ret = 1;
  do {
    if (n & 1)
      ret = ret * a % mod;
    a = 1LL * a * a % mod;
  } while (n >>= 1);
  return ret;
}

template <typename T> T GCD(T a, T b) { return b ? GCD(b, a % b) : a; }
template <typename T> T LCM(T a, T b) { return a / GCD(a, b) * b; }

template <typename T1, typename T2>
bool CompareBySecond(pair<T1, T2> a, pair<T1, T2> b) {
  return a.second != b.second ? a.second < b.second : a.first < b.first;
}
// counterclockwise from 12 o'clock direction
template <typename T> bool CompareBySlope(pair<T, T> a, pair<T, T> b) {
  if (a.first * b.first < 0)
    return a.first < b.first;
  if (a.first == 0)
    return a.second >= 0 || b.first > 0;
  if (b.first == 0)
    return b.second < 0 && a.first < 0;
  return a.second * b.first < a.first * b.second;
}

/* -------- <templates end> -------- */

struct path {
  int sz, to;
  char c;

  path(int sz, int to, char c = 0) : sz(sz), to(to), c(c) {}
};

path zero(3e5, -1);

void calc(Graph<int> &pos, vector<path> &dp, int pre, int i, char c) {
  rep(j, 26) {
    auto itr = lower_bound(ALL(pos[j]), pre);

    if (*itr == -1) {
      if (zero.sz < dp[i].sz + 1)
        continue;
      if (zero.sz == dp[i].sz + 1 && zero.c < c)
        continue;

      zero = {dp[i].sz + 1, i, c};
    }

    else if (itr != pos[j].end()) {
      if (dp[*itr].sz < dp[i].sz + 1)
        continue;
      if (dp[*itr].sz == dp[i].sz + 1 && dp[*itr].c < c)
        continue;

      dp[*itr] = {dp[i].sz + 1, i, c};
    }
  }
}

void solve() {
  string s;
  cin >> s;

  Graph<int> pos(26, vi(1, -1));
  int sz = s.size();
  rep(i, sz) { pos[s[i] - 'a'].emplace_back(i); }

  vector<path> dp(sz + 1, path(3e5, sz));
  dp[sz] = {0, -1};

  rep(i, 26) {
    if (pos[i].back() == -1) {
      cout << char('a' + i) << endl;
      return;
    }

    dp[sz].c = 'a' + i;
    calc(pos, dp, pos[i].back(), sz, dp[sz].c);
  }

  zero.to = sz;

  rrep(i, sz) {
    auto pre = lower_bound(ALL(pos[s[i] - 'a']), i);
    --pre;
    calc(pos, dp, *pre, i, s[i]);
  }

  int idx = zero.to;
  string ans;
  ans.push_back(zero.c);

  while (idx != sz) {
    ans.push_back(dp[idx].c);
    idx = dp[idx].to;
  }

  cout << ans << endl;
}

/* -------- <programs end> -------- */

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  solve();
  return 0;
}