Problem p02616 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <iostream>
#include <vector>

#define ll long long
ll MAX = 1000000000 + 7;

using namespace std;

ll modF(ll a) {
  if (a < 0)
    return modF((a % MAX) + MAX);
  else
    return (a % MAX);
}

int main() {
  ll t = 1;
  while (t--) {
    ll n, k, i, a, po = 0, ni = 0, ans = 1;
    vector<ll> vcp, vcn;
    cin >> n >> k;
    for (i = 0; i < n; i++) {
      cin >> a;
      if (a < 0) {
        a = -a;
        ni++;
        vcn.push_back(a);
      } else {
        po++;
        vcp.push_back(a);
      }
    }
    if (po > 0)
      sort(vcp.begin(), vcp.end());
    if (ni > 0)
      sort(vcn.begin(), vcn.end());

    if (po == 0) {
      if (k % 2 == 0) {
        for (i = 0; i < k; i++) {
          ans = ans * (-vcn[ni - 1]);
          ans = modF(ans);
          ni--;
        }
      } else {
        for (i = 0; i < k; i++) {
          ans = ans * (-vcn[i]);
          ans = modF(ans);
        }
      }
    } else if (ni == 0) {
      for (i = 0; i < k; i++) {
        ans = ans * vcp[po - 1];
        ans = modF(ans);
        po--;
      }
    } else {
      if (k % 2 == 1) {
        ans = vcp[po - 1];
        po--;
        k--;
      }
      while (k > 1 && (po >= 2 || ni >= 2)) {
        ll propo = 0;
        if (po >= 2)
          propo = vcp[po - 1] * vcp[po - 2];
        ll proni = 0;
        if (ni >= 2)
          proni = vcn[ni - 1] * vcn[ni - 2];

        if (propo > proni) {
          propo = modF(propo);
          ans = ans * propo;
          ans = modF(ans);
          po = po - 2;
        } else {
          proni = modF(proni);
          ans = ans * proni;
          ans = modF(ans);
          ni = ni - 2;
        }
        k = k - 2;
      }
      while (k > 0) {
        if (po > 0) {
          ans = ans * vcp[po - 1];
          ans = modF(ans);
          po--;
          k--;
        } else if (ni > 0) {
          ans = ans * (-vcn[ni - 1]);
          ans = modF(ans);
          ni--;
          k--;
        }
      }
    }
    cout << ans << endl;
  }
  return 0;
}

Error: 0

Fixed submission:

#include <algorithm>
#include <assert.h>
#include <bitset>
#include <iostream>
#include <map>
#include <math.h>
#include <memory.h>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <stdio.h>
#include <string>
#include <time.h>
#include <unordered_map>
#include <vector>

using namespace std;

const int mod = 1e9 + 7;

int a[111111];

int cmp(int a, int b) { return abs(b) > abs(a); }

int main() {
  ios::sync_with_stdio(0);
  int sn = 0;
  int n, k;
  cin >> n >> k;
  assert(1 <= k && k <= n && n <= 1e5);
  sn += n;
  assert(sn <= 5e5);
  bool zero = 0;
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
    assert(-1e9 <= a[i] && a[i] <= 1e9);
    zero |= (a[i] == 0);
  }
  sort(a, a + n, cmp);

  //        for(int i = 0; i < n; ++i) {
  //            cout << a[i] << " ";
  //            if (i == n-k-1)
  //                cout << "\\n";
  //        }
  //        cout << "\\n";

  sn = 1;
  vector<int> nums;
  int gneg = -1e9 - 1, gpos = 1e9 + 1;
  for (int i = 0, id = n - 1; i < k; ++i, id--) {
    if (a[id] < 0)
      sn *= -1;
    nums.push_back(a[id]);
    if (a[id] < 0)
      gneg = max(gneg, a[id]);
    else
      gpos = min(gpos, a[id]);
  }

  if (sn > 0) {
    int ans = 1;
    for (auto x : nums)
      ans = (ans * 1ll * x) % mod;
    cout << (ans + mod) % mod << "\\n";
  } else {
    int mn = 1, mx = -1;
    for (int i = 0; i < n - k; ++i) {
      mn = min(mn, a[i]);
      mx = max(mx, a[i]);
    }

    // cout << gneg << " " << mx << "\\n";
    // cout << gpos << " " << mn << "\\n";

    bool g1 = gneg != -1e-9 - 1 && mx != -1;
    bool g2 = gpos != 1e9 + 1 && mn != 1;
    if (g1 && g2) {
      int val, repl;
      if (mx * 1ll * gpos > mn * 1ll * gneg)
        val = gneg, repl = mx;
      else
        val = gpos, repl = mn;

      for (int i = 0; i < k; ++i)
        if (nums[i] == val) {
          nums[i] = repl;
          break;
        }
      int ans = 1;
      for (auto x : nums)
        ans = (ans * 1ll * x) % mod;
      cout << (ans + mod) % mod << "\\n";
    } else if (g1) {
      for (int i = 0; i < k; ++i)
        if (nums[i] == gneg) {
          nums[i] = mx;
          break;
        }
      int ans = 1;
      for (auto x : nums)
        ans = (ans * 1ll * x) % mod;
      cout << (ans + mod) % mod << "\\n";
    } else if (g2) {
      for (int i = 0; i < k; ++i)
        if (nums[i] == gpos) {
          nums[i] = mn;
          break;
        }
      int ans = 1;
      for (auto x : nums)
        ans = (ans * 1ll * x) % mod;
      cout << (ans + mod) % mod << "\\n";
    } else {
      if (zero)
        cout << "0\\n";
      else {
        int ans = 1;
        for (int i = 0; i < k; ++i)
          ans = (ans * 1ll * a[i]) % mod;
        cout << (ans + mod) % mod << "\\n";
      }
    }
  }
  return 0;
}