Problem p03393 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using Edge = int;
using Graph = vector<vector<Edge>>;
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define SORT(v) sort((v).begin(), (v).end())
#define RSORT(v) sort((v).rbegin(), (v).rend())
const ll MOD = 1000000007;
const ll nmax = 8;
const ll INF = 1e9;
const int MAX = 510000;
bool graph[nmax][nmax];
long long fac[MAX], finv[MAX], inv[MAX];

void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

ll COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;

  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

vector<vector<ll>> dist = vector<vector<ll>>(nmax, vector<ll>(nmax, INF));
struct SegmentTree {
private:
  ll n;
  vector<ll> node;

public:
  SegmentTree(vector<ll> v) {
    ll sz = v.size();
    n = 1;
    while (n < sz) {
      n *= 2;
    }
    node.resize(2 * n - 1, INF);

    for (ll i = 0; i < sz; i++) {
      node[i + n - 1] = v[i];
    }

    for (ll i = n - 2; i >= 0; i--) {
      node[i] = min(node[2 * i + 1], node[2 * i + 2]);
    }
  }
  void update(ll x, ll val) {
    x += (n - 1);
    node[x] = val;
    while (x > 0) {
      x = (x - 1) / 2;
      node[x] = min(node[2 * x + 1], node[2 * x + 2]);
    }
  }

  // findは半開区間で考える
  ll find(ll a, ll b, ll k = 0, ll l = 0, ll r = -1) {
    if (r < 0)
      r = n;

    if (r <= a || b <= l)
      return INF;

    if (a <= l && r <= b)
      return node[k];

    ll vl = find(a, b, 2 * k + 1, l, (l + r) / 2);
    ll vr = find(a, b, 2 * k + 2, (l + r) / 2, r);
    return min(vl, vr);
  }
};
void warshall_floyd(ll n) {
  for (size_t i = 0; i < n; i++) {
    for (size_t j = 0; j < n; j++) {
      for (size_t k = 0; k < n; k++) {
        dist[j][k] = min(dist[j][k], dist[j][i] + dist[i][k]);
      }
    }
  }
}

class UnionFind {
public:
  vector<ll> Parent;

  UnionFind(ll N) { Parent = vector<ll>(N, -1); }
  ll find(ll A) {
    if (Parent[A] < 0)
      return A;
    return Parent[A] = find(Parent[A]);
  }

  ll size(ll A) { return -Parent[find(A)]; }

  bool Union(ll A, ll B) {
    A = find(A);
    B = find(B);
    if (A == B) {
      return false;
    }
    if (size(A) < size(B))
      swap(A, B);

    Parent[A] += Parent[B];
    Parent[B] = A;

    return true;
  }
};

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll lcm(ll a, ll b) {
  ll g = gcd(a, b);
  return a / g * b;
}

ll mulMod(ll a, ll b) { return (((a % MOD) * (b % MOD)) % MOD); }

ll powMod(ll a, ll p) {
  if (p == 0) {
    return 1;
  } else if (p % 2 == 0) {
    ll half = powMod(a, p / 2);
    return mulMod(half, half);
  } else {
    return mulMod(powMod(a, p - 1), a);
  }
}

ll ceil(ll a, ll b) { return (a + b - 1) / b; }

vector<ll> tsort(Graph G) {
  ll N = G.size();
  vector<ll> in(N);
  for (auto &&edges : G) {
    for (auto &&edge : edges) {
      in[edge]++;
    }
  }

  queue<int> que;

  for (int i = 0; i < N; i++) {
    if (in[i] == 0) {
      que.push(i);
    }
  }
  int cnt = 0;
  vector<ll> ans;
  while (!que.empty()) {
    int v = que.front();
    que.pop();
    ans.push_back(v);
    for (auto &&next : G[v]) {

      in[next]--;
      if (in[next] == 0) {
        que.push(next);
      }
    }
  }
  return ans;
}
Graph G(100);
void treeDFS(int from, int current, int dist, int &maxDist, int &maxVertex) {
  if (dist > maxDist) {
    maxDist = dist;
    maxVertex = current;
  }

  for (auto to : G[current]) {
    if (to == from)
      continue;
    treeDFS(current, to, dist + 1, maxDist, maxVertex);
  }
}

pair<int, int> getTreeDiameter() {
  int start = 0, end = 0, maxDist = 0;
  treeDFS(-1, start, 0, maxDist, end);
  start = end, end = 0, maxDist = 0;
  treeDFS(-1, start, 0, maxDist, end);
  return make_pair(start, end);
}

void solve(std::string S) {
  vector<char> a(26);
  for (char c = 'a'; c <= 'z'; c++) {
    a[c - 'a'] = c;
  }

  if (S == "zyxwvutsrqponmlkjihgfedcba") {
    cout << -1 << endl;
    return;
  }
  if (S.size() == 26) {
    do {
      string tmp;
      for (int i = 0; i < 26; i++) {
        tmp += a[i];
      }
      if (tmp > S) {
        for (int i = 0; i < 26; i++) {
          if (tmp[i] == S[i]) {
            cout << tmp[i];
            continue;
          } else {
            cout << tmp[i] << endl;
            return;
          }
        }
      }

    } while (next_permutation(a.begin(), a.end()));
  } else {
    for (char c = 'a'; c <= 'z'; c++) {
      if (find(S.begin(), S.end(), c) == S.end()) {
        cout << S << c << endl;
        return;
      }
    }
  }
}

int main() {
  std::string S;
  std::cin >> S;
  solve(S);
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using Edge = int;
using Graph = vector<vector<Edge>>;
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define SORT(v) sort((v).begin(), (v).end())
#define RSORT(v) sort((v).rbegin(), (v).rend())
const ll MOD = 1000000007;
const ll nmax = 8;
const ll INF = 1e9;
const int MAX = 510000;
bool graph[nmax][nmax];
long long fac[MAX], finv[MAX], inv[MAX];

void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

ll COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;

  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

vector<vector<ll>> dist = vector<vector<ll>>(nmax, vector<ll>(nmax, INF));
struct SegmentTree {
private:
  ll n;
  vector<ll> node;

public:
  SegmentTree(vector<ll> v) {
    ll sz = v.size();
    n = 1;
    while (n < sz) {
      n *= 2;
    }
    node.resize(2 * n - 1, INF);

    for (ll i = 0; i < sz; i++) {
      node[i + n - 1] = v[i];
    }

    for (ll i = n - 2; i >= 0; i--) {
      node[i] = min(node[2 * i + 1], node[2 * i + 2]);
    }
  }
  void update(ll x, ll val) {
    x += (n - 1);
    node[x] = val;
    while (x > 0) {
      x = (x - 1) / 2;
      node[x] = min(node[2 * x + 1], node[2 * x + 2]);
    }
  }

  // findは半開区間で考える
  ll find(ll a, ll b, ll k = 0, ll l = 0, ll r = -1) {
    if (r < 0)
      r = n;

    if (r <= a || b <= l)
      return INF;

    if (a <= l && r <= b)
      return node[k];

    ll vl = find(a, b, 2 * k + 1, l, (l + r) / 2);
    ll vr = find(a, b, 2 * k + 2, (l + r) / 2, r);
    return min(vl, vr);
  }
};
void warshall_floyd(ll n) {
  for (size_t i = 0; i < n; i++) {
    for (size_t j = 0; j < n; j++) {
      for (size_t k = 0; k < n; k++) {
        dist[j][k] = min(dist[j][k], dist[j][i] + dist[i][k]);
      }
    }
  }
}

class UnionFind {
public:
  vector<ll> Parent;

  UnionFind(ll N) { Parent = vector<ll>(N, -1); }
  ll find(ll A) {
    if (Parent[A] < 0)
      return A;
    return Parent[A] = find(Parent[A]);
  }

  ll size(ll A) { return -Parent[find(A)]; }

  bool Union(ll A, ll B) {
    A = find(A);
    B = find(B);
    if (A == B) {
      return false;
    }
    if (size(A) < size(B))
      swap(A, B);

    Parent[A] += Parent[B];
    Parent[B] = A;

    return true;
  }
};

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll lcm(ll a, ll b) {
  ll g = gcd(a, b);
  return a / g * b;
}

ll mulMod(ll a, ll b) { return (((a % MOD) * (b % MOD)) % MOD); }

ll powMod(ll a, ll p) {
  if (p == 0) {
    return 1;
  } else if (p % 2 == 0) {
    ll half = powMod(a, p / 2);
    return mulMod(half, half);
  } else {
    return mulMod(powMod(a, p - 1), a);
  }
}

ll ceil(ll a, ll b) { return (a + b - 1) / b; }

vector<ll> tsort(Graph G) {
  ll N = G.size();
  vector<ll> in(N);
  for (auto &&edges : G) {
    for (auto &&edge : edges) {
      in[edge]++;
    }
  }

  queue<int> que;

  for (int i = 0; i < N; i++) {
    if (in[i] == 0) {
      que.push(i);
    }
  }
  int cnt = 0;
  vector<ll> ans;
  while (!que.empty()) {
    int v = que.front();
    que.pop();
    ans.push_back(v);
    for (auto &&next : G[v]) {

      in[next]--;
      if (in[next] == 0) {
        que.push(next);
      }
    }
  }
  return ans;
}
Graph G(100);
void treeDFS(int from, int current, int dist, int &maxDist, int &maxVertex) {
  if (dist > maxDist) {
    maxDist = dist;
    maxVertex = current;
  }

  for (auto to : G[current]) {
    if (to == from)
      continue;
    treeDFS(current, to, dist + 1, maxDist, maxVertex);
  }
}

pair<int, int> getTreeDiameter() {
  int start = 0, end = 0, maxDist = 0;
  treeDFS(-1, start, 0, maxDist, end);
  start = end, end = 0, maxDist = 0;
  treeDFS(-1, start, 0, maxDist, end);
  return make_pair(start, end);
}

void solve(std::string S) {
  vector<char> a(26);
  for (char c = 'a'; c <= 'z'; c++) {
    a[c - 'a'] = c;
  }

  if (S == "zyxwvutsrqponmlkjihgfedcba") {
    cout << -1 << endl;
    return;
  }
  if (S.size() == 26) {
    string T = S;
    next_permutation(S.begin(), S.end());
    for (int i = 0; i < S.size(); i++) {
      cout << S[i];
      if (S[i] != T[i])
        break;
    }
    cout << endl;
  } else {
    for (char c = 'a'; c <= 'z'; c++) {
      if (find(S.begin(), S.end(), c) == S.end()) {
        cout << S << c << endl;
        return;
      }
    }
  }
}

int main() {
  std::string S;
  std::cin >> S;
  solve(S);
  return 0;
}