Problem p03629 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define ll long long
#define P pair<int, int>
#define fi first
#define se second
#define rep(i, n) for (int i = 0; i < n; i++)
#define all(v) v.begin(), v.end()
#define pb push_back
#define eb emplace_back
template <class T> void chmax(T &a, T b) {
  if (a < b)
    a = b;
}
template <class T> void chmin(T &a, T b) {
  if (a > b)
    a = b;
}
constexpr int INF = 1000000000000000000;
constexpr int mod = 1000000007;
constexpr double eps = 1e-8;
const double pi = acos(-1);
int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};
int Random(int mi, int ma) {
  random_device rnd;
  mt19937 mt(rnd()); // 32bit
  //[mi,ma]
  uniform_int_distribution<int> engine(mi, ma);
  return engine(mt);
}
int gcd(int a, int b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
int lcm(int a, int b) { return a / gcd(a, b) * b; }
bool prime(int a) {
  if (a == 1)
    return false;
  for (int i = 2; i * i <= a; i++) {
    if (a % i == 0)
      return false;
  }
  return true;
}
int modpow(int a, int b) {
  if (b == 0)
    return 1;
  if (b % 2)
    return modpow(a, b - 1) * a % mod;
  int memo = modpow(a, b / 2);
  return memo * memo % mod;
}
vector<int> kaijo, invkaijo;
void init_fact(int n) {
  kaijo.resize(n + 1);
  invkaijo.resize(n + 1);
  kaijo[0] = 1;
  for (int i = 1; i <= n; i++) {
    kaijo[i] = kaijo[i - 1] * i;
    kaijo[i] %= mod;
  }
  rep(i, n + 1) invkaijo[i] = modpow(kaijo[i], mod - 2);
}
int comb(int a, int b) {
  if (a < b)
    return 0;
  if (a < 0 || b < 0)
    return 0;
  return kaijo[a] * modpow(kaijo[a - b], mod - 2) % mod *
         modpow(kaijo[b], mod - 2) % mod;
}
int inv(int x) {
  x = modpow(x, mod - 2);
  return x;
}
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  string s, ans = "";
  cin >> s;
  int n = s.size(), dp[200010], pos = 0;
  vector<int> v[26];
  rep(i, n) v[s[i] - 'a'].pb(i);
  rep(i, 26) v[i].pb(n);
  fill(dp, dp + n, INF);
  dp[n] = 1;
  for (int i = n - 1; i >= 0; i--)
    rep(j, 26) chmin(dp[i], dp[*lower_bound(all(v[j]), i) + 1] + 1);
  while (pos <= n) {
    rep(i, 26) {
      int nxt = *lower_bound(all(v[i]), pos);
      if (dp[pos] == dp[nxt + 1] + 1) {
        ans += 'a' + i;
        pos = nxt + 1;
        break;
      }
    }
  }
  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define ll long long
#define P pair<int, int>
#define fi first
#define se second
#define rep(i, n) for (int i = 0; i < n; i++)
#define all(v) v.begin(), v.end()
#define pb push_back
#define eb emplace_back
template <class T> void chmax(T &a, T b) {
  if (a < b)
    a = b;
}
template <class T> void chmin(T &a, T b) {
  if (a > b)
    a = b;
}
constexpr int INF = 1000000000000000000;
constexpr int mod = 1000000007;
constexpr double eps = 1e-8;
const double pi = acos(-1);
int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};
int Random(int mi, int ma) {
  random_device rnd;
  mt19937 mt(rnd()); // 32bit
  //[mi,ma]
  uniform_int_distribution<int> engine(mi, ma);
  return engine(mt);
}
int gcd(int a, int b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
int lcm(int a, int b) { return a / gcd(a, b) * b; }
bool prime(int a) {
  if (a == 1)
    return false;
  for (int i = 2; i * i <= a; i++) {
    if (a % i == 0)
      return false;
  }
  return true;
}
int modpow(int a, int b) {
  if (b == 0)
    return 1;
  if (b % 2)
    return modpow(a, b - 1) * a % mod;
  int memo = modpow(a, b / 2);
  return memo * memo % mod;
}
vector<int> kaijo, invkaijo;
void init_fact(int n) {
  kaijo.resize(n + 1);
  invkaijo.resize(n + 1);
  kaijo[0] = 1;
  for (int i = 1; i <= n; i++) {
    kaijo[i] = kaijo[i - 1] * i;
    kaijo[i] %= mod;
  }
  rep(i, n + 1) invkaijo[i] = modpow(kaijo[i], mod - 2);
}
int comb(int a, int b) {
  if (a < b)
    return 0;
  if (a < 0 || b < 0)
    return 0;
  return kaijo[a] * modpow(kaijo[a - b], mod - 2) % mod *
         modpow(kaijo[b], mod - 2) % mod;
}
int inv(int x) {
  x = modpow(x, mod - 2);
  return x;
}
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  string s, ans = "";
  cin >> s;
  int n = s.size(), dp[200010], pos = 0;
  vector<int> v[26];
  rep(i, n) v[s[i] - 'a'].pb(i);
  rep(i, 26) v[i].pb(n);
  fill(dp, dp + n, INF);
  dp[n] = 1, dp[n + 1] = 0;
  for (int i = n - 1; i >= 0; i--)
    rep(j, 26) chmin(dp[i], dp[*lower_bound(all(v[j]), i) + 1] + 1);
  while (pos <= n) {
    rep(i, 26) {
      int nxt = *lower_bound(all(v[i]), pos);
      if (dp[pos] == dp[nxt + 1] + 1) {
        ans += 'a' + i;
        pos = nxt + 1;
        break;
      }
    }
  }
  cout << ans << endl;
  return 0;
}