Problem p02647 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 200005;

ll n, k;
array<ll, N> old, nw;
bitset<N> used;

int main() {
  cin.sync_with_stdio(0);
  cin.tie(0);
  cin >> n >> k;
  vector<ll> a(n);
  for (auto &i : a)
    cin >> i;

  for (int i = 0; i < n; ++i) {
    old[max(0ll, i - a[i])]++;
    old[min(n, i + a[i] + 1)]--;
  }

  for (int i = 1; i < k; ++i) {
    ll psum = 0;
    used.reset();
    for (int j = 0; j < n; ++j) {
      if (i % 2) {
        psum += old[j];
        // cout << i << ' ' << j << ' ' << psum << '\\n';
        ll mn = max(0ll, j - psum), mx = min(n, j + psum + 1);
        if (!used[j]) {
          used[j] = 1;
          nw[j] = 0;
        }
        if (!used[mn]) {
          used[mn] = 1;
          nw[mn] = 0;
        }
        if (!used[mx]) {
          used[mx] = 1;
          nw[mx] = 0;
        }

        // cout << j << ' ' << mn << ' ' << mx << '\\n';
        nw[mn]++;
        nw[mx]--;
      } else {
        psum += nw[j];
        // cout << i << ' ' << j << ' ' << psum << '\\n';
        ll mn = max(0ll, j - psum), mx = min(n, j + psum + 1);
        if (!used[j]) {
          used[j] = 1;
          old[j] = 0;
        }
        if (!used[mn]) {
          used[mn] = 1;
          old[mn] = 0;
        }
        if (!used[mx]) {
          used[mx] = 1;
          old[mx] = 0;
        }

        // cout << j << ' ' << mn << ' ' << mx << '\\n';
        // cout << old[mn] << ' ' << old[mx] << '\\n';
        old[mn]++;
        old[mx]--;
      }
    }

    if (nw == old)
      break;
  }

  if (k % 2) {
    ll psum = 0;
    for (int i = 0; i < n; ++i) {
      psum += old[i];
      cout << psum << ' ';
    }
  } else {
    ll psum = 0;
    for (int i = 0; i < n; ++i) {
      psum += nw[i];
      cout << psum << ' ';
    }
  }
}


Error: WA

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 200005;

ll n, k;
array<ll, N> old, nw;
bitset<N> used;

int main() {
  cin.sync_with_stdio(0);
  cin.tie(0);
  cin >> n >> k;
  vector<ll> a(n);
  for (auto &i : a)
    cin >> i;

  for (int i = 0; i < n; ++i) {
    old[max(0ll, i - a[i])]++;
    old[min(n, i + a[i] + 1)]--;
  }

  for (int i = 1; i < k; ++i) {
    ll psum = 0;
    used.reset();
    for (int j = 0; j < n; ++j) {
      if (i % 2) {
        psum += old[j];
        // cout << i << ' ' << j << ' ' << psum << '\\n';
        ll mn = max(0ll, j - psum), mx = min(n, j + psum + 1);
        if (!used[mn]) {
          used[mn] = 1;
          nw[mn] = 0;
        }
        if (!used[mx]) {
          used[mx] = 1;
          nw[mx] = 0;
        }

        // cout << j << ' ' << mn << ' ' << mx << '\\n';
        nw[mn]++;
        nw[mx]--;
      } else {
        psum += nw[j];
        // cout << i << ' ' << j << ' ' << psum << '\\n';
        ll mn = max(0ll, j - psum), mx = min(n, j + psum + 1);
        if (!used[j]) {
          used[j] = 1;
          old[j] = 0;
        }
        if (!used[mn]) {
          used[mn] = 1;
          old[mn] = 0;
        }
        if (!used[mx]) {
          used[mx] = 1;
          old[mx] = 0;
        }

        // cout << j << ' ' << mn << ' ' << mx << '\\n';
        // cout << old[mn] << ' ' << old[mx] << '\\n';
        old[mn]++;
        old[mx]--;
      }
    }

    if (nw == old)
      break;
  }

  if (k % 2) {
    ll psum = 0;
    for (int i = 0; i < n; ++i) {
      psum += old[i];
      cout << psum << ' ';
    }
  } else {
    ll psum = 0;
    for (int i = 0; i < n; ++i) {
      psum += nw[i];
      cout << psum << ' ';
    }
  }
}