Problem p02623 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <bits/stdc++.h>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <string>
#include <vector>
using namespace std;

const double PI = 3.141592653589793238463;
#define pb push_back
#define pf push_front
#define sz(v) ((int)(v).size());
#define all(v) ((v).begin()), ((v).end())
typedef unsigned long long ull;
typedef long long ll;

typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<ull> vul;

typedef pair<int, int> pi;
// 2 7 3 4 9

void printVec(vul v1) {
  for (int i = 0; i < v1.size(); i++)
    cout << v1[i] << " ";
  cout << endl;
}
ll rec(ll r, ll c) {

  if (r <= 1 || c <= 1) {
    return 1;
  }

  return rec(r - 1, c) + rec(r, c - 1);
}

bool bin_search(ull target, vul V) {

  ll low = 0;
  ll high = V.size() - 1;
  while (low <= high) {
    ll mid = (low + high) / 2;
    if (target == V[mid]) {
      return true;
    }
    if (target < V[mid]) {
      high = mid - 1;
    } else {
      low = mid + 1;
    }
  }
  return false;
}
int main() {
  ios::sync_with_stdio(false);
  cout.tie(NULL);
  cin.tie(NULL);
  ll n, m, k;
  cin >> n >> m >> k;
  vul a(n, 0);
  vul b(m, 0);

  for (ll i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (ll i = 0; i < m; i++) {
    cin >> b[i];
  }

  vul A(n + 1, 0);
  vul B(m + 1, 0);

  for (ll i = 0; i < n; i++) {
    A[i + 1] = A[i] + a[i];
  }
  for (ll i = 0; i < m; i++) {
    B[i + 1] = B[i] + b[i];
  }

  ll books = 0, A_books = 0, B_books = 0;
  ull sum = 0;

  vector<ull>::iterator lower, upper;
  for (ull i = 0; i <= n; i++) {

    ll t = k - A[i];
    if (t < 0) {
      continue;
    }
    lower = lower_bound(B.begin(), B.end(), k - A[i]);
    B_books = lower - B.begin();
    if (B[B_books] > k - A[i] || B_books > m) {
      B_books--;
    }
    // B_books--;
    sum = max(sum, B_books + i);
  }
  cout << sum << endl;
  return 0;
}


Error: WA

Fixed submission:

#include <algorithm>
#include <bits/stdc++.h>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <string>
#include <vector>
using namespace std;

const double PI = 3.141592653589793238463;
#define pb push_back
#define pf push_front
#define sz(v) ((int)(v).size());
#define all(v) ((v).begin()), ((v).end())
typedef unsigned long long ull;
typedef long long ll;

typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<ull> vul;

typedef pair<int, int> pi;
// 2 7 3 4 9

void printVec(vul v1) {
  for (int i = 0; i < v1.size(); i++)
    cout << v1[i] << " ";
  cout << endl;
}
ll rec(ll r, ll c) {

  if (r <= 1 || c <= 1) {
    return 1;
  }

  return rec(r - 1, c) + rec(r, c - 1);
}

bool bin_search(ull target, vul V) {

  ll low = 0;
  ll high = V.size() - 1;
  while (low <= high) {
    ll mid = (low + high) / 2;
    if (target == V[mid]) {
      return true;
    }
    if (target < V[mid]) {
      high = mid - 1;
    } else {
      low = mid + 1;
    }
  }
  return false;
}
int main() {
  ios::sync_with_stdio(false);
  cout.tie(NULL);
  cin.tie(NULL);
  ll n, m, k;
  cin >> n >> m >> k;
  vul a(n, 0);
  vul b(m, 0);

  for (ll i = 0; i < n; i++) {
    cin >> b[i];
  }
  for (ll i = 0; i < m; i++) {
    cin >> b[i];
  }

  vul A(n + 1, 0);
  vul B(m + 1, 0);

  for (ll i = 0; i < n; i++) {
    A[i + 1] = A[i] + a[i];
  }
  for (ll i = 0; i < m; i++) {
    B[i + 1] = B[i] + b[i];
  }

  ll books = 0, A_books = 0, B_books = 0;
  ull sum = 0;

  vector<ull>::iterator lower, upper;
  for (ull i = 0; i <= n; i++) {

    ll t = k - A[i];
    if (t < 0) {
      continue;
    }
    lower = lower_bound(B.begin(), B.end(), k - A[i]);
    B_books = lower - B.begin();
    if (B[B_books] > k - A[i] || B_books > m) {
      B_books--;
    }
    // B_books--;
    sum = max(sum, B_books + i);
  }
  cout << sum << endl;
  return 0;
}