Problem p02623 (C++)

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

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPr(i, n) for (int i = (n)-1; i >= 0; --i)
#define FORq(i, m, n) for (int i = (m); i <= (n); ++i)
#define FORqr(i, m, n) for (int i = (n); i >= (m); --i)
#define MP make_pair
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x, 0, sizeof(x))
#define FILL(x, y) memset(x, y, sizeof(x))
#define MM(x) memset(x, -1, sizeof(x))
#define ALL(x) (x).begin(), (x).end()
#define DB(x) cerr << #x << " = " << x << endl
#define DB2(x, y)                                                              \
  cerr << "(" << #x << ", " << #y << ") = (" << x << ", " << y << ")\n";
#define DEBUG                                                                  \
  int x12345;                                                                  \
  cin >> x12345;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
const long long MOD = 1e9 + 7;
///////////////////////////////////////////////
// for template
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
///////////////////////////////////////////////
/// 🍈( '-' 🍈 |AC|
int main() {
  ll N, M, K;
  cin >> N >> M >> K;
  vector<ll> A(N), B(M), Sa(N + 1), Sb(M + 1);
  Sa[0] = 0;
  Sb[0] = 0;
  REP(i, N) {
    cin >> A[i];
    Sa[i + 1] = A[i] + Sa[i];
  }

  REP(i, M) {
    cin >> B[i];
    Sb[i + 1] = B[i] + Sb[i];
  }

  ll ans = 0;
  Sb.push_back(1e10);

  FORq(i, 0, N) {
    ll Ncost = Sa[i];
    if (Ncost > K)
      continue;
    auto it = upper_bound(ALL(Sb), K - Ncost);
    ll bcount = it - Sb.begin() - 1;
    ll nans = (ll)i + bcount;
    DB2(i, bcount);
    chmax(ans, nans);
  }

  // REP(i,N+1){
  //     DB2(i,Sa[i]);
  // }

  // REP(i,M+1){
  //     DB2(i,Sb[i]);
  // }

  cout << ans << endl;
}

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

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPr(i, n) for (int i = (n)-1; i >= 0; --i)
#define FORq(i, m, n) for (int i = (m); i <= (n); ++i)
#define FORqr(i, m, n) for (int i = (n); i >= (m); --i)
#define MP make_pair
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x, 0, sizeof(x))
#define FILL(x, y) memset(x, y, sizeof(x))
#define MM(x) memset(x, -1, sizeof(x))
#define ALL(x) (x).begin(), (x).end()
#define DB(x) cerr << #x << " = " << x << endl
#define DB2(x, y)                                                              \
  cerr << "(" << #x << ", " << #y << ") = (" << x << ", " << y << ")\n";
#define DEBUG                                                                  \
  int x12345;                                                                  \
  cin >> x12345;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
const long long MOD = 1e9 + 7;
///////////////////////////////////////////////
// for template
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
///////////////////////////////////////////////
/// 🍈( '-' 🍈 |AC|
int main() {
  ll N, M, K;
  cin >> N >> M >> K;
  vector<ll> A(N), B(M), Sa(N + 1), Sb(M + 1);
  Sa[0] = 0;
  Sb[0] = 0;
  REP(i, N) {
    cin >> A[i];
    Sa[i + 1] = A[i] + Sa[i];
  }

  REP(i, M) {
    cin >> B[i];
    Sb[i + 1] = B[i] + Sb[i];
  }

  ll ans = 0;
  Sb.push_back(1e10);

  FORq(i, 0, N) {
    ll Ncost = Sa[i];
    if (Ncost > K)
      continue;
    auto it = upper_bound(ALL(Sb), K - Ncost);
    ll bcount = it - Sb.begin() - 1;
    ll nans = (ll)i + bcount;
    // DB2(i,bcount);
    chmax(ans, nans);
  }

  // REP(i,N+1){
  //     DB2(i,Sa[i]);
  // }

  // REP(i,M+1){
  //     DB2(i,Sb[i]);
  // }

  cout << ans << endl;
}