Problem p02856 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;

#define INF_LL (int64)1e18
#define INF (int32)1e9
#define REP(i, n) for (int64 i = 0; i < (n); i++)
#define FOR(i, a, b) for (int64 i = (a); i < (b); i++)
#define all(x) x.begin(), x.end()
#define fs first
#define sc second

using int32 = int_fast32_t;
using uint32 = uint_fast32_t;
using int64 = int_fast64_t;
using uint64 = uint_fast64_t;
using PII = pair<int32, int32>;
using PLL = pair<int64, int64>;

const double eps = 1e-10;

template <typename A, typename B> inline void chmin(A &a, B b) {
  if (a > b)
    a = b;
}
template <typename A, typename B> inline void chmax(A &a, B b) {
  if (a < b)
    a = b;
}

template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename U, typename... V>
typename enable_if<is_same<T, U>::value != 0>::type fill_v(U &u, const V... v) {
  u = U(v...);
}

template <typename T, typename U, typename... V>
typename enable_if<is_same<T, U>::value == 0>::type fill_v(U &u, const V... v) {
  for (auto &e : u)
    fill_v<T>(e, v...);
}

PLL solve(int64 x, int64 c) {
  if (x == 0)
    return PLL(x, c - 1);
  int64 cnt = 0, now = x, i;
  for (i = 1; i < c; i++) {
    now += x;
    cnt++;
    while (now > 9) {
      now = now / 10 + now % 10;
      cnt++;
    }
    if (now == x)
      break;
  }
  if (i == c - 1)
    return PLL(now, cnt);
  int64 loop = cnt;
  cnt = 0;
  while (c >= loop) {
    cnt += c / (i + 1) * loop;
    c = c / (i + 1) + c % (i + 1);
  }
  now = x;
  REP(i, c - 1) {
    cnt++;
    now = now + x;
    while (now > 9) {
      cnt++;
      now = now / 10 + now % 10;
    }
  }
  return PLL(now, cnt);
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int64 M;
  cin >> M;
  vector<int64> d(M), c(M), a(M);
  int64 res = 0;
  REP(i, M) {
    cin >> d[i] >> c[i];
    PLL ret = solve(d[i], c[i]);
    //    cout << ret.sc << " " << ret.fs << endl;
    res += ret.sc;
    a[i] = ret.fs;
    //	  cout << ret.fs << " " << ret.sc << endl;
  }
  int64 now = a[0];
  FOR(i, 1, M) {
    now += a[i];
    res++;
    while (now > 9) {
      now = (now % 10) + now / 10;
      res++;
    }
  }
  cout << res << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

#define INF_LL (int64)1e18
#define INF (int32)1e9
#define REP(i, n) for (int64 i = 0; i < (n); i++)
#define FOR(i, a, b) for (int64 i = (a); i < (b); i++)
#define all(x) x.begin(), x.end()
#define fs first
#define sc second

using int32 = int_fast32_t;
using uint32 = uint_fast32_t;
using int64 = int_fast64_t;
using uint64 = uint_fast64_t;
using PII = pair<int32, int32>;
using PLL = pair<int64, int64>;

const double eps = 1e-10;

template <typename A, typename B> inline void chmin(A &a, B b) {
  if (a > b)
    a = b;
}
template <typename A, typename B> inline void chmax(A &a, B b) {
  if (a < b)
    a = b;
}

template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename U, typename... V>
typename enable_if<is_same<T, U>::value != 0>::type fill_v(U &u, const V... v) {
  u = U(v...);
}

template <typename T, typename U, typename... V>
typename enable_if<is_same<T, U>::value == 0>::type fill_v(U &u, const V... v) {
  for (auto &e : u)
    fill_v<T>(e, v...);
}

PLL solve(int64 x, int64 c) {
  if (x == 0)
    return PLL(x, c - 1);
  int64 cnt = 0, now = x, i;
  for (i = 1; i < c; i++) {
    now += x;
    cnt++;
    while (now > 9) {
      now = now / 10 + now % 10;
      cnt++;
    }
    if (now == x)
      break;
  }
  if (i == c - 1)
    return PLL(now, cnt);
  int64 loop = cnt;
  cnt = 0;
  while (c >= i + 1) {
    cnt += c / (i + 1) * loop;
    c = c / (i + 1) + c % (i + 1);
  }
  now = x;
  REP(i, c - 1) {
    cnt++;
    now = now + x;
    while (now > 9) {
      cnt++;
      now = now / 10 + now % 10;
    }
  }
  return PLL(now, cnt);
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int64 M;
  cin >> M;
  vector<int64> d(M), c(M), a(M);
  int64 res = 0;
  REP(i, M) {
    cin >> d[i] >> c[i];
    PLL ret = solve(d[i], c[i]);
    //    cout << ret.sc << " " << ret.fs << endl;
    res += ret.sc;
    a[i] = ret.fs;
    //	  cout << ret.fs << " " << ret.sc << endl;
  }
  int64 now = a[0];
  FOR(i, 1, M) {
    now += a[i];
    res++;
    while (now > 9) {
      now = (now % 10) + now / 10;
      res++;
    }
  }
  cout << res << endl;
}