Problem p04017 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>
#define X first
#define Y second
#define pb push_back
#define rep(X, Y) for (int(X) = 0; (X) < (Y); ++(X))
#define reps(X, S, Y) for (int(X) = S; (X) < (Y); ++(X))
#define rrep(X, Y) for (int(X) = (Y)-1; (X) >= 0; --(X))
#define repe(X, Y) for ((X) = 0; (X) < (Y); ++(X))
#define peat(X, Y) for (; (X) < (Y); ++(X))
#define all(X) (X).begin(), (X).end()
#define rall(X) (X).rbegin(), (X).rend()
#define eb emplace_back
#define UNIQUE(X) (X).erase(unique(all(X)), (X).end())

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
template <class T> using vv = vector<vector<T>>;
template <class T> ostream &operator<<(ostream &os, const vector<T> &t) {
  os << "{";
  rep(i, t.size()) { os << t[i] << ","; }
  os << "}" << endl;
  return os;
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &t) {
  return os << "(" << t.first << "," << t.second << ")";
}
template <class T> inline bool MX(T &l, const T &r) {
  return l < r ? l = r, 1 : 0;
}
template <class T> inline bool MN(T &l, const T &r) {
  return l > r ? l = r, 1 : 0;
}
const ll MOD = 1e9 + 7;

int db[20][112345];

int main() {
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(0);
  int n;
  cin >> n;
  vector<int> x(n);
  rep(i, n) cin >> x[i];
  int len;
  cin >> len;
  int r = 0;
  rep(l, n) {
    while (r + 1 < n && x[r + 1] - x[l] <= len)
      ++r;
    db[l][0] = r;
  }
  rep(i, 19) rep(j, n) db[j][i + 1] = db[db[j][i]][i];
  // rep(j,20){rep(i,n) cout<<db[i][j]<<",";cout<<endl;}
  int q;
  cin >> q;
  int a, b;
  while (q--) {
    cin >> a >> b;
    --a;
    --b;
    if (a > b)
      swap(a, b);
    int re = 0;
    rrep(i, 20) if (db[a][i] < b) {
      a = db[a][i];
      re += 1 << i;
    }
    cout << re + 1 << endl;
  }
  return 0;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>
#define X first
#define Y second
#define pb push_back
#define rep(X, Y) for (int(X) = 0; (X) < (Y); ++(X))
#define reps(X, S, Y) for (int(X) = S; (X) < (Y); ++(X))
#define rrep(X, Y) for (int(X) = (Y)-1; (X) >= 0; --(X))
#define repe(X, Y) for ((X) = 0; (X) < (Y); ++(X))
#define peat(X, Y) for (; (X) < (Y); ++(X))
#define all(X) (X).begin(), (X).end()
#define rall(X) (X).rbegin(), (X).rend()
#define eb emplace_back
#define UNIQUE(X) (X).erase(unique(all(X)), (X).end())

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
template <class T> using vv = vector<vector<T>>;
template <class T> ostream &operator<<(ostream &os, const vector<T> &t) {
  os << "{";
  rep(i, t.size()) { os << t[i] << ","; }
  os << "}" << endl;
  return os;
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &t) {
  return os << "(" << t.first << "," << t.second << ")";
}
template <class T> inline bool MX(T &l, const T &r) {
  return l < r ? l = r, 1 : 0;
}
template <class T> inline bool MN(T &l, const T &r) {
  return l > r ? l = r, 1 : 0;
}
const ll MOD = 1e9 + 7;

int db[112345][20];

int main() {
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(0);
  int n;
  cin >> n;
  vector<int> x(n);
  rep(i, n) cin >> x[i];
  int len;
  cin >> len;
  int r = 0;
  rep(l, n) {
    while (r + 1 < n && x[r + 1] - x[l] <= len)
      ++r;
    db[l][0] = r;
  }
  rep(i, 19) rep(j, n) db[j][i + 1] = db[db[j][i]][i];
  // rep(j,20){rep(i,n) cout<<db[i][j]<<",";cout<<endl;}
  int q;
  cin >> q;
  int a, b;
  while (q--) {
    cin >> a >> b;
    --a;
    --b;
    if (a > b)
      swap(a, b);
    int re = 0;
    rrep(i, 20) if (db[a][i] < b) {
      a = db[a][i];
      re += 1 << i;
    }
    cout << re + 1 << endl;
  }
  return 0;
}