Problem p02623 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define rep(i, start, end) for (long long i = start; i < end; ++i)
#define repreverse(i, start, end) for (long long i = start; i >= end; --i)
#define all(x) (x).begin(), (x).end()
#define len(x) ((long long)(x).size())
#define lcm(a, b) ((a) / __gcd((a), (b)) * (b))
using namespace std;
using ll = long long;
using ld = long double;
using vll = vector<ll>;
using vllvll = vector<vll>;
using pll = pair<ll, ll>;
template <class T> void print1d(T x, ll n = -1) {
  if (n == -1)
    n = x.size();
  rep(i, 0, n) { cout << x[i] << ' '; }
  cout << '\n';
}
template <class T> void print2d(T x, ll r = -1, ll c = -1) {
  if (r == -1)
    r = x.size();
  if (c == -1)
    c = x[0].size();
  rep(i, 0, r) print1d(x[i], c);
}
template <class T, class U> bool haskey(T mp, U key) {
  return mp.find(key) != mp.end();
}
template <class T, class U> bool isin(T el, U container) {
  return find(all(container), el) != container.end();
}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool even(T n) { return !(n & 1); }
template <class T> bool odd(T n) { return n & 1; }
template <class T> ld deg2rad(T deg) { return M_PI * deg / 180.0; }
template <class T> ld rad2deg(T rad) { return 180.0 * rad / M_PI; }
ll intpow(ll a, ll n) {
  ll p = 1;
  while (n) {
    if (n & 1)
      p *= a;
    a *= a;
    n >>= 1;
  }
  return p;
}
const long double pi = M_PI;
const long long big = 1LL << 50;
const long long inf = 1LL << 60;
const long long mod = 1e9 + 7;

int main() {
  ll N, M, K;
  cin >> N >> M >> K;
  vll A(N), B(M);
  rep(i, 0, N) cin >> A[i];
  rep(i, 0, M) cin >> B[i];
  vll acc1(N + 1), acc2(M + 1);
  rep(i, 1, N + 1) acc1[i] += acc1[i - 1] + A[i - 1];
  rep(i, 1, M + 1) acc2[i] += acc2[i - 1] + B[i - 1];

  ll ans = 0;
  rep(i, 0, N) {
    if (K - acc1[i + 1] < 0)
      break;
    ll pos = upper_bound(all(acc2), K - acc1[i + 1]) - acc2.begin() - 1;
    ll tmp = max(pos, 0LL);
    chmax(ans, i + 1 + tmp);
  }

  rep(i, 0, N) {
    if (K - acc2[i + 1] < 0)
      break;
    ll pos = upper_bound(all(acc1), K - acc2[i + 1]) - acc1.begin() - 1;
    ll tmp = max(pos, 0LL);
    chmax(ans, i + 1 + tmp);
  }

  cout << ans << endl;
}

Fixed submission:
#include <bits/stdc++.h>
#define rep(i, start, end) for (long long i = start; i < end; ++i)
#define repreverse(i, start, end) for (long long i = start; i >= end; --i)
#define all(x) (x).begin(), (x).end()
#define len(x) ((long long)(x).size())
#define lcm(a, b) ((a) / __gcd((a), (b)) * (b))
using namespace std;
using ll = long long;
using ld = long double;
using vll = vector<ll>;
using vllvll = vector<vll>;
using pll = pair<ll, ll>;
template <class T> void print1d(T x, ll n = -1) {
  if (n == -1)
    n = x.size();
  rep(i, 0, n) { cout << x[i] << ' '; }
  cout << '\n';
}
template <class T> void print2d(T x, ll r = -1, ll c = -1) {
  if (r == -1)
    r = x.size();
  if (c == -1)
    c = x[0].size();
  rep(i, 0, r) print1d(x[i], c);
}
template <class T, class U> bool haskey(T mp, U key) {
  return mp.find(key) != mp.end();
}
template <class T, class U> bool isin(T el, U container) {
  return find(all(container), el) != container.end();
}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool even(T n) { return !(n & 1); }
template <class T> bool odd(T n) { return n & 1; }
template <class T> ld deg2rad(T deg) { return M_PI * deg / 180.0; }
template <class T> ld rad2deg(T rad) { return 180.0 * rad / M_PI; }
ll intpow(ll a, ll n) {
  ll p = 1;
  while (n) {
    if (n & 1)
      p *= a;
    a *= a;
    n >>= 1;
  }
  return p;
}
const long double pi = M_PI;
const long long big = 1LL << 50;
const long long inf = 1LL << 60;
const long long mod = 1e9 + 7;

int main() {
  ll N, M, K;
  cin >> N >> M >> K;
  vll A(N), B(M);
  rep(i, 0, N) cin >> A[i];
  rep(i, 0, M) cin >> B[i];
  vll acc1(N + 1), acc2(M + 1);
  rep(i, 1, N + 1) acc1[i] += acc1[i - 1] + A[i - 1];
  rep(i, 1, M + 1) acc2[i] += acc2[i - 1] + B[i - 1];

  ll ans = 0;
  rep(i, 0, N) {
    if (K - acc1[i + 1] < 0)
      break;
    ll pos = upper_bound(all(acc2), K - acc1[i + 1]) - acc2.begin() - 1;
    ll tmp = max(pos, 0LL);
    chmax(ans, i + 1 + tmp);
  }

  rep(i, 0, M) {
    if (K - acc2[i + 1] < 0)
      break;
    ll pos = upper_bound(all(acc1), K - acc2[i + 1]) - acc1.begin() - 1;
    ll tmp = max(pos, 0LL);
    chmax(ans, i + 1 + tmp);
  }

  cout << ans << endl;
}