Problem p03629 (C++)

Buggy submission (status: Runtime Error):
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define double long double
using datas = pair<ll, ll>;
using ddatas = pair<double, double>;
using tdata = pair<ll, datas>;
using vec = vector<ll>;
using mat = vector<vec>;
using pvec = vector<datas>;
using pmat = vector<pvec>;
#define For(i, a, b) for (i = a; i < (ll)b; i++)
#define bFor(i, a, b) for (i = (ll)a - 1; i >= (ll)b; i--)
#define rep(i, N) For(i, 0, N)
#define rep1(i, N) For(i, 1, N)
#define brep(i, N) bFor(i, N, 0)
#define all(v) (v).begin(), (v).end()
#define allr(v) (v).rbegin(), (v).rend()
#define vsort(v) sort(all(v))
#define vrsort(v) sort(allr(v))
#define endl "\n"
#define pb push_back
#define print(v) cout << v << endl
#define printyes cout << "Yes" << endl
#define printno cout << "No" << endl
#define printYES cout << "YES" << endl
#define printNO cout << "NO" << endl
#define output(v)                                                              \
  do {                                                                         \
    bool f = 0;                                                                \
    for (auto outi : v) {                                                      \
      cout << (f ? " " : "") << outi;                                          \
      f = 1;                                                                   \
    }                                                                          \
    cout << endl;                                                              \
  } while (0)
const ll mod = 1000000007;
const ll inf = 1LL << 26;
const double PI = acos(-1);
const double eps = 1e-9;
template <class T> inline bool chmax(T &a, T b) {
  bool x = a < b;
  if (x)
    a = b;
  return x;
}
template <class T> inline bool chmin(T &a, T b) {
  bool x = a > b;
  if (x)
    a = b;
  return x;
}

void startupcpp() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}

double distance(ddatas x, ddatas y) {
  double a = x.first - y.first, b = x.second - y.second;
  return sqrt(a * a + b * b);
}

ll modinv(ll a) {
  ll b = mod, u = 1, v = 0, t;
  while (b) {
    t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  return (u + mod) % mod;
}

ll moddevide(ll a, ll b) { return (a * modinv(b)) % mod; }

vec modncrlistp, modncrlistm;

ll modncr(ll n, ll r) {
  ll i, size = modncrlistp.size();
  if (size <= n) {
    modncrlistp.resize(n + 1);
    modncrlistm.resize(n + 1);
    if (!size) {
      modncrlistp[0] = modncrlistm[0] = 1;
      size++;
    }
    For(i, size, n + 1) {
      modncrlistp[i] = modncrlistp[i - 1] * i % mod;
      modncrlistm[i] = modinv(modncrlistp[i]);
    }
  }
  return modncrlistp[n] * modncrlistm[r] % mod * modncrlistm[n - r] % mod;
}

ll modpow(ll a, ll n) {
  ll res = 1;
  while (n) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

ll gcd(ll a, ll b) {
  if (!b)
    return a;
  return (a % b == 0) ? b : gcd(b, a % b);
}
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

ll countdigits(ll n) {
  ll ans = 0;
  while (n) {
    n /= 10;
    ans++;
  }
  return ans;
}

ll sumdigits(ll n) {
  ll ans = 0;
  while (n) {
    ans += n % 10;
    n /= 10;
  }
  return ans;
}
int main() {
  // startupcpp();
  // ll test;cin>>test;while(test--){
  ll i, j = 0, N, K;
  string s, ans = "";
  cin >> s;
  N = s.size();
  vec term(1, N), ch(26, 0);
  brep(i, N) {
    j |= 1LL << (s[i] - 'a');
    if (j + 1 == inf) {
      j = 0;
      term.pb(i);
    }
  }
  K = term.size();
  rep(i, term[K - 1]) { ++ch[s[i] - 'a']; }
  N = 0;
  while (ch[N])
    ++N;
  ans += 'a' + N;
  reverse(all(term));
  rep1(i, K) {
    rep(j, 26) ch[j] = 0;
    j = term[i - 1];
    while (s[++j] - 'a' != N)
      ;
    while (j < term[i])
      ++ch[s[j++] - 'a'];
    N = 0;
    while (ch[N])
      ++N;
    ans += 'a' + N;
  }
  print(ans);
  return 0;
}

Fixed submission:
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define double long double
using datas = pair<ll, ll>;
using ddatas = pair<double, double>;
using tdata = pair<ll, datas>;
using vec = vector<ll>;
using mat = vector<vec>;
using pvec = vector<datas>;
using pmat = vector<pvec>;
#define For(i, a, b) for (i = a; i < (ll)b; i++)
#define bFor(i, a, b) for (i = (ll)a - 1; i >= (ll)b; i--)
#define rep(i, N) For(i, 0, N)
#define rep1(i, N) For(i, 1, N)
#define brep(i, N) bFor(i, N, 0)
#define all(v) (v).begin(), (v).end()
#define allr(v) (v).rbegin(), (v).rend()
#define vsort(v) sort(all(v))
#define vrsort(v) sort(allr(v))
#define endl "\n"
#define pb push_back
#define print(v) cout << v << endl
#define printyes cout << "Yes" << endl
#define printno cout << "No" << endl
#define printYES cout << "YES" << endl
#define printNO cout << "NO" << endl
#define output(v)                                                              \
  do {                                                                         \
    bool f = 0;                                                                \
    for (auto outi : v) {                                                      \
      cout << (f ? " " : "") << outi;                                          \
      f = 1;                                                                   \
    }                                                                          \
    cout << endl;                                                              \
  } while (0)
const ll mod = 1000000007;
const ll inf = 1LL << 26;
const double PI = acos(-1);
const double eps = 1e-9;
template <class T> inline bool chmax(T &a, T b) {
  bool x = a < b;
  if (x)
    a = b;
  return x;
}
template <class T> inline bool chmin(T &a, T b) {
  bool x = a > b;
  if (x)
    a = b;
  return x;
}

void startupcpp() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}

double distance(ddatas x, ddatas y) {
  double a = x.first - y.first, b = x.second - y.second;
  return sqrt(a * a + b * b);
}

ll modinv(ll a) {
  ll b = mod, u = 1, v = 0, t;
  while (b) {
    t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  return (u + mod) % mod;
}

ll moddevide(ll a, ll b) { return (a * modinv(b)) % mod; }

vec modncrlistp, modncrlistm;

ll modncr(ll n, ll r) {
  ll i, size = modncrlistp.size();
  if (size <= n) {
    modncrlistp.resize(n + 1);
    modncrlistm.resize(n + 1);
    if (!size) {
      modncrlistp[0] = modncrlistm[0] = 1;
      size++;
    }
    For(i, size, n + 1) {
      modncrlistp[i] = modncrlistp[i - 1] * i % mod;
      modncrlistm[i] = modinv(modncrlistp[i]);
    }
  }
  return modncrlistp[n] * modncrlistm[r] % mod * modncrlistm[n - r] % mod;
}

ll modpow(ll a, ll n) {
  ll res = 1;
  while (n) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

ll gcd(ll a, ll b) {
  if (!b)
    return a;
  return (a % b == 0) ? b : gcd(b, a % b);
}
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

ll countdigits(ll n) {
  ll ans = 0;
  while (n) {
    n /= 10;
    ans++;
  }
  return ans;
}

ll sumdigits(ll n) {
  ll ans = 0;
  while (n) {
    ans += n % 10;
    n /= 10;
  }
  return ans;
}
int main() {
  // startupcpp();
  // ll test;cin>>test;while(test--){
  ll i, j = 0, N, K;
  string s, ans = "";
  cin >> s;
  N = s.size();
  vec term(1, N), ch(26, 0);
  brep(i, N) {
    j |= 1LL << (s[i] - 'a');
    if (j + 1 == inf) {
      j = 0;
      term.pb(i);
    }
  }
  K = term.size();
  rep(i, term[K - 1]) { ++ch[s[i] - 'a']; }
  N = 0;
  while (ch[N])
    ++N;
  ans += 'a' + N;
  reverse(all(term));
  rep1(i, K) {
    rep(j, 26) ch[j] = 0;
    j = term[i - 1];
    while (s[j++] - 'a' != N)
      ;
    while (j < term[i])
      ++ch[s[j++] - 'a'];
    N = 0;
    while (ch[N])
      ++N;
    ans += 'a' + N;
  }
  print(ans);
  return 0;
}