Problem p02599 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

template <class Abel> struct BIT {
  vector<Abel> dat[2];
  Abel UNITY_SUM = 0; // to be set

  /* [1, n] */
  BIT(int n) { init(n); }
  void init(int n) {
    for (int iter = 0; iter < 2; ++iter)
      dat[iter].assign(n + 1, UNITY_SUM);
  }

  /* a, b are 1-indexed, [a, b) */
  inline void sub_add(int p, int a, Abel x) {
    for (int i = a; i < (int)dat[p].size(); i += i & -i)
      dat[p][i] = dat[p][i] + x;
  }
  inline void add(int a, int b, Abel x) {
    sub_add(0, a, x * -(a - 1));
    sub_add(1, a, x);
    sub_add(0, b, x * (b - 1));
    sub_add(1, b, x * (-1));
  }

  /* a is 1-indexed, [a, b) */
  inline Abel sub_sum(int p, int a) {
    Abel res = UNITY_SUM;
    for (int i = a; i > 0; i -= i & -i)
      res = res + dat[p][i];
    return res;
  }
  inline Abel sum(int a, int b) {
    return sub_sum(0, b - 1) + sub_sum(1, b - 1) * (b - 1) - sub_sum(0, a - 1) -
           sub_sum(1, a - 1) * (a - 1);
  }
  /* debug */
  void print() {
    for (int i = 1; i < (int)dat[0].size(); ++i)
      cout << sum(i, i + 1) << ",";
    cout << endl;
  }
};

int main() {
  int N;
  cin >> N;
  int Q;
  cin >> Q;
  vector<int> a(N);
  for (int i = 0; i < N; ++i)
    cin >> a[i];

  vector<int> lefts(Q), rights(Q), ids(Q);
  for (int i = 0; i < Q; ++i) {
    cin >> lefts[i] >> rights[i];
    --lefts[i];
    ids[i] = i;
  }
  sort(ids.begin(), ids.end(),
       [&](int i, int j) { return rights[i] < rights[j]; });

  BIT<int> bit(N + 5);
  vector<int> prev(1100000, -1);
  vector<int> res(Q, 0);
  int r = 0;
  for (auto i : ids) {
    for (; r < rights[i]; ++r) {
      bit.add(prev[a[r]] + 2, r + 2, 1);
      prev[a[r]] = r;
    }
    int tmp = bit.sum(lefts[i] + 1, lefts[i] + 2);
    res[i] = max(res[i], tmp);
  }
  for (int i = 0; i < Q; ++i)
    cout << res[i] << endl;
}

/*
cd ~/Desktop/Programming/AtCoder/practice/abc174/abc174_f && sh
shellscripts/test_cpp.sh
*/

Error: WA

Fixed submission:

#include <algorithm>
#include <cmath>
#include <iostream>
#include <numeric>
#include <vector>
using namespace std;

struct Mo {
  vector<int> left, right, index; // the interval's left, right, index
  vector<bool> v;
  int window;
  int nl, nr, ptr;

  Mo(int n) : window((int)sqrt(n)), nl(0), nr(0), ptr(0), v(n, false) {}

  /* push */
  void push(int l, int r) { left.push_back(l), right.push_back(r); }

  /* sort intervals */
  void build() {
    index.resize(left.size());
    iota(index.begin(), index.end(), 0);
    sort(begin(index), end(index), [&](int a, int b) {
      if (left[a] / window != left[b] / window)
        return left[a] < left[b];
      return bool((right[a] < right[b]) ^ (left[a] / window % 2));
    });
  }

  /* extend-shorten */
  void extend_shorten(int id) {
    v[id].flip();
    if (v[id])
      insert(id);
    else
      erase(id);
  }

  /* next id of interval */
  int next() {
    if (ptr == index.size())
      return -1;
    int id = index[ptr];
    while (nl > left[id])
      extend_shorten(--nl);
    while (nr < right[id])
      extend_shorten(nr++);
    while (nl < left[id])
      extend_shorten(nl++);
    while (nr > right[id])
      extend_shorten(--nr);
    return index[ptr++];
  }

  /* insert, erase (to be set appropriately) */
  void insert(int id);
  void erase(int id);
};

int N, Q;
int A[300001];
int res[200001];
int cnt[1000001];
int num_kind = 0;

void Mo::insert(int id) {
  int val = A[id];
  if (cnt[val] == 0)
    ++num_kind;
  ++cnt[val];
}

void Mo::erase(int id) {
  int val = A[id];
  --cnt[val];
  if (cnt[val] == 0)
    --num_kind;
}

int main() {
  scanf("%d%d", &N, &Q);
  for (int i = 0; i < N; i++)
    scanf("%d", &A[i]);
  Mo mo(N);
  for (int i = 0; i < Q; i++) {
    int l, r;
    scanf("%d %d", &l, &r);
    mo.push(--l, r);
  }
  mo.build();
  for (int i = 0; i < Q; i++)
    res[mo.next()] = num_kind;
  for (int i = 0; i < Q; i++)
    printf("%d\\n", res[i]);
}

/*
cd ~/Desktop/Programming/AtCoder/contest/abc174/abc174_f && sh
shellscripts/test_cpp.sh cd
~/Desktop/Programming/AtCoder/contest/abc174/abc174_f && sh
shellscripts/etest_cpp.sh cd
~/Desktop/Programming/AtCoder/contest/abc174/abc174_f && sh
shellscripts/submit_cpp.sh
*/