Problem p02346 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cassert>
#include <cmath>
#include <iostream>
#include <string>
#include <vector>
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define REP(i, n) FOR(i, 0, n)
#define rep(i, n) FOR(i, 0, n)
#define DEBUG(x) cout << #x << ": " << x << endl
#define vint vector<int>
#define vdouble vector<double>
#define vstring vector<string>
using namespace std;

#include <map>
#include <queue>
#include <set>

typedef long long ll;
typedef unsigned long long ull;

// const int MAX_N = 1000000;
const int INFTY = (1 << 21); // 2097152
// const ll INFTY = (1LL << 60);
const ll MD = 1000000007LL;
// fprintf(stderr, "%d %lld \n", x, xll);

// segtree
const int MAX_N = 200000; // 1 << 20; // > 10^6

class SegTree {

public:
  int n;
  vector<ll> dat;
  ll dummy = 0; // (1 << 31) - 1;

  SegTree(int _n) { init(_n); }

  void init(int n_) {
    dat = vector<ll>(2 * MAX_N - 1);
    n = 1;
    while (n < n_)
      n *= 2;

    // initialize
    rep(i, 2 * n - 1) dat[i] = dummy;
  }

  void update(int k, ll a) {
    // update from bottom
    k += n - 1;
    dat[k] += a;
    while (k > 0) {
      k = (k - 1) / 2;
      dat[k] = dat[k * 2 + 1] + dat[k * 2 + 2];
    }
  }

  // sum of [a, b)
  ll query(int a, int b) { return _query(a, b, 0, 0, n); }

  // sum of [a, b)
  // node k, corresponding [l, r)
  ll _query(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return dummy;
    if (a <= l && r <= b)
      return dat[k];
    else {
      int vl = _query(a, b, k * 2 + 1, l, (l + r) / 2);
      int vr = _query(a, b, k * 2 + 2, (l + r) / 2, r);
      return vl + vr;
    }
  }
};

int N, Q;
int main() {
  cin >> N >> Q;

  SegTree ST(N);

  rep(q, Q) {
    int c, x, y;
    cin >> c >> x >> y;

    if (c == 0) {
      // update
      x--;
      ST.update(x, y);
    } else {
      // find
      x--;
      y--;
      cout << ST.query(x, y + 1) << endl;
    }
  }

  rep(i, ST.n * 2 - 1) { fprintf(stderr, "%d ", ST.dat[i]); }
  fprintf(stderr, "\n");
}

Fixed submission:
#include <algorithm>
#include <cassert>
#include <cmath>
#include <iostream>
#include <string>
#include <vector>
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define REP(i, n) FOR(i, 0, n)
#define rep(i, n) FOR(i, 0, n)
#define DEBUG(x) cout << #x << ": " << x << endl
#define vint vector<int>
#define vdouble vector<double>
#define vstring vector<string>
using namespace std;

#include <map>
#include <queue>
#include <set>

typedef long long ll;
typedef unsigned long long ull;

// const int MAX_N = 1000000;
const int INFTY = (1 << 21); // 2097152
// const ll INFTY = (1LL << 60);
const ll MD = 1000000007LL;
// fprintf(stderr, "%d %lld \n", x, xll);

// segtree
const int MAX_N = 200000; // 1 << 20; // > 10^6

class SegTree {

public:
  int n;
  vector<ll> dat;
  ll dummy = 0; // (1 << 31) - 1;

  SegTree(int _n) { init(_n); }

  void init(int n_) {
    dat = vector<ll>(2 * MAX_N - 1);
    n = 1;
    while (n < n_)
      n *= 2;

    // initialize
    rep(i, 2 * n - 1) dat[i] = dummy;
  }

  void update(int k, ll a) {
    // update from bottom
    k += n - 1;
    dat[k] += a;
    while (k > 0) {
      k = (k - 1) / 2;
      dat[k] = dat[k * 2 + 1] + dat[k * 2 + 2];
    }
  }

  // sum of [a, b)
  ll query(int a, int b) { return _query(a, b, 0, 0, n); }

  // sum of [a, b)
  // node k, corresponding [l, r)
  ll _query(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return dummy;
    if (a <= l && r <= b)
      return dat[k];
    else {
      int vl = _query(a, b, k * 2 + 1, l, (l + r) / 2);
      int vr = _query(a, b, k * 2 + 2, (l + r) / 2, r);
      return vl + vr;
    }
  }
};

int N, Q;
int main() {
  cin >> N >> Q;

  SegTree ST(N);

  rep(q, Q) {
    int c, x, y;
    cin >> c >> x >> y;

    if (c == 0) {
      // update
      x--;
      ST.update(x, y);
    } else {
      // find
      x--;
      y--;
      cout << ST.query(x, y + 1) << endl;
    }
  }

  // rep(i, ST.n * 2 - 1){fprintf(stderr, "%d ", ST.dat[i]); } fprintf(stderr,
  // "\n");
}