Problem p02265 (C++) — buggy submission, verdict: Runtime Error

// #define DEBUGRUN

#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;

#include <list>

// size, push_back, pop_back, insert, erase

const int MAX_N = 3000000;
char cmd[MAX_N];
int arg[MAX_N];

int main() {

  int N;
  cin >> N;

  rep(i, N) {

    // string command;
    // cin >> command;
    char command[20];
    scanf("%s", command);

#ifdef DEBUGRUN
    cerr << "X";
#endif
    // cerr << "X";
    if (command[0] == 'i') {
      cmd[i] = 'I';
      cin >> arg[i];
    } else if (command[6] == 'F') {
      cmd[i] = 'F';
    } else if (command[6] == 'L') {
      cmd[i] = 'L';
    } else {
      cmd[i] = 'D';
      cin >> arg[i];
    }
  }

  list<int> lst;
  rep(i, N) {
    if (cmd[i] == 'I') {
      lst.push_front(arg[i]);
    }
    if (cmd[i] == 'D') {
      for (list<int>::iterator it = lst.begin(); it != lst.end(); it++) {
        if (*it == arg[i]) {
          lst.erase(it);
          break;
        }
      }
    }
    if (cmd[i] == 'F') {
      lst.pop_front();
    }
    if (cmd[i] == 'L') {
      lst.pop_back();
    }
  }

  for (list<int>::iterator it = lst.begin(); it != lst.end(); it++) {
    if (it != lst.begin()) {
      cout << ' ';
    }
    cout << *it;
  }
  cout << endl;
}

Error: 0
XXXXXXX

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;

#include <list>

// size, push_back, pop_back, insert, erase

const int MAX_N = 3000000;
char cmd[MAX_N];
int arg[MAX_N];

int main() {

  int N;
  cin >> N;

  rep(i, N) {

    // string command;
    // cin >> command;
    char command[20];
    scanf("%s", command);

#ifdef DEBUG
    cerr << "X";
#endif
    // cerr << "X";
    if (command[0] == 'i') {
      cmd[i] = 'I';
      cin >> arg[i];
    } else if (command[6] == 'F') {
      cmd[i] = 'F';
    } else if (command[6] == 'L') {
      cmd[i] = 'L';
    } else {
      cmd[i] = 'D';
      cin >> arg[i];
    }
  }

  list<int> lst;
  rep(i, N) {
    if (cmd[i] == 'I') {
      lst.push_front(arg[i]);
    }
    if (cmd[i] == 'D') {
      for (list<int>::iterator it = lst.begin(); it != lst.end(); it++) {
        if (*it == arg[i]) {
          lst.erase(it);
          break;
        }
      }
    }
    if (cmd[i] == 'F') {
      lst.pop_front();
    }
    if (cmd[i] == 'L') {
      lst.pop_back();
    }
  }

  for (list<int>::iterator it = lst.begin(); it != lst.end(); it++) {
    if (it != lst.begin()) {
      cout << ' ';
    }
    cout << *it;
  }
  cout << endl;
}