Problem p04050 (C++)

Buggy submission (status: Runtime Error):
// {{{ by shik
#include <bits/stdc++.h>
#include <unistd.h>
#define SZ(x) ((int)(x).size())
#define ALL(x) begin(x), end(x)
#define REP(i, n) for (int i = 0; i < int(n); i++)
#define REP1(i, a, b) for (int i = (a); i <= int(b); i++)
#define FOR(it, c) for (auto it = (c).begin(); it != (c).end(); it++)
#define MP make_pair
#define PB push_back
using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
typedef vector<int> VI;

#ifdef SHIK
template <typename T> void _dump(const char *s, T &&head) {
  cerr << s << "=" << head << endl;
}

template <typename T, typename... Args>
void _dump(const char *s, T &&head, Args &&...tail) {
  int c = 0;
  while (*s != ',' || c != 0) {
    if (*s == '(' || *s == '[' || *s == '{')
      c++;
    if (*s == ')' || *s == ']' || *s == '}')
      c--;
    cerr << *s++;
  }
  cerr << "=" << head << ", ";
  _dump(s + 1, tail...);
}

#define dump(...)                                                              \
  do {                                                                         \
    fprintf(stderr, "%s:%d - ", __PRETTY_FUNCTION__, __LINE__);                \
    _dump(#__VA_ARGS__, __VA_ARGS__);                                          \
  } while (0)

template <typename Iter> ostream &_out(ostream &s, Iter b, Iter e) {
  s << "[";
  for (auto it = b; it != e; it++)
    s << (it == b ? "" : " ") << *it;
  s << "]";
  return s;
}

template <typename A, typename B>
ostream &operator<<(ostream &s, const pair<A, B> &p) {
  return s << "(" << p.first << "," << p.second << ")";
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &c) {
  return _out(s, ALL(c));
}
template <typename T, size_t N>
ostream &operator<<(ostream &s, const array<T, N> &c) {
  return _out(s, ALL(c));
}
template <typename T> ostream &operator<<(ostream &s, const set<T> &c) {
  return _out(s, ALL(c));
}
template <typename A, typename B>
ostream &operator<<(ostream &s, const map<A, B> &c) {
  return _out(s, ALL(c));
}
#else
#define dump(...)
#endif

template <typename T> void _R(T &x) { cin >> x; }
void _R(int &x) { scanf("%d", &x); }
void _R(long long &x) { scanf("%" PRId64, &x); }
void _R(double &x) { scanf("%lf", &x); }
void _R(char &x) { scanf(" %c", &x); }
void _R(char *x) { scanf("%s", x); }

void R() {}
template <typename T, typename... U> void R(T &head, U &...tail) {
  _R(head);
  R(tail...);
}

template <typename T> void _W(const T &x) { cout << x; }
void _W(const int &x) { printf("%d", x); }
template <typename T> void _W(const vector<T> &x) {
  for (auto i = x.cbegin(); i != x.cend(); i++) {
    if (i != x.cbegin())
      putchar(' ');
    _W(*i);
  }
}

void W() {}
template <typename T, typename... U> void W(const T &head, const U &...tail) {
  _W(head);
  putchar(sizeof...(tail) ? ' ' : '\n');
  W(tail...);
}

#ifdef SHIK
#define FILEIO(...)
#else
#define FILEIO(name)                                                           \
  do {                                                                         \
    freopen(name ".in", "r", stdin);                                           \
    freopen(name ".out", "w", stdout);                                         \
  } while (0)
#endif

// }}}

struct DJS {
  vector<int> fa, sz;
  void init(int n) {
    n++; // be nice for 1-index usage
    fa.resize(n);
    sz.resize(n);
    for (int i = 0; i < n; i++)
      sz[fa[i] = i] = 1;
  }
  int f(int x) { return x == fa[x] ? x : fa[x] = f(fa[x]); }
  void u(int a, int b) {
    a = f(a);
    b = f(b);
    if (a == b)
      return;
    if (sz[a] > sz[b])
      swap(a, b);
    fa[a] = b;
    sz[b] += sz[a];
  }
} djs;

bool cmp(int x, int y) { return MP(x % 2, x) < MP(y % 2, y); }

const int M = 110;
int n, m, a[M], bn, b[M];

bool chk() {
  djs.init(n);
  int s = 0;
  REP(i, m) {
    REP1(j, 1, a[i]) djs.u(s + j, s + a[i] - j + 1);
    s += a[i];
  }
  if (s != n)
    return 0;
  s = 0;
  REP(i, bn) {
    REP1(j, 1, b[i]) djs.u(s + j, s + b[i] - j + 1);
    s += b[i];
  }
  if (s != n)
    return 0;
  REP1(i, 1, n) if (djs.f(i) != djs.f(1)) return 0;
  return 1;
}

void output() {
  W(VI(a, a + m));
  W(bn);
  W(VI(b, b + bn));
  assert(chk());
}

int main() {
  R(n, m);
  REP(i, m) R(a[i]);
  sort(a, a + m, cmp);
  dump(VI(a, a + m));
  int odd = 0;
  REP(i, m) if (a[i] % 2 == 1) odd++;
  if (odd == 0) {
    REP(i, m) b[bn++] = a[i];
    b[0]--;
    b[bn++] = 1;
    output();
  } else if (odd == 1) {
    if (m == 1) {
      if (a[0] == 1) {
        b[bn++] = 1;
      } else {
        b[bn++] = a[0] - 1;
        b[bn++] = 1;
      }
    } else {
      REP(i, m) b[bn++] = a[i];
      b[0]--;
      b[bn - 1]++;
    }
    output();
  } else if (odd == 2) {
    if (m == 2) {
      if (n == 2) {
        b[bn++] = 2;
        output();
      } else {
        b[bn++] = n - 2;
        b[bn++] = 2;
        if (chk())
          output();
        else {
          swap(b[0], b[1]);
          if (chk())
            output();
        }
      }
    } else {
      swap(a[0], a[m - 2]);
      REP1(i, 1, m - 1) b[bn++] = a[i];
      b[0] += a[0] - 1;
      b[bn - 1]++;
      output();
    }
  } else {
    W("Impossible");
  }
  return 0;
}


Fixed submission:
// {{{ by shik
#include <bits/stdc++.h>
#include <unistd.h>
#define SZ(x) ((int)(x).size())
#define ALL(x) begin(x), end(x)
#define REP(i, n) for (int i = 0; i < int(n); i++)
#define REP1(i, a, b) for (int i = (a); i <= int(b); i++)
#define FOR(it, c) for (auto it = (c).begin(); it != (c).end(); it++)
#define MP make_pair
#define PB push_back
using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
typedef vector<int> VI;

#ifdef SHIK
template <typename T> void _dump(const char *s, T &&head) {
  cerr << s << "=" << head << endl;
}

template <typename T, typename... Args>
void _dump(const char *s, T &&head, Args &&...tail) {
  int c = 0;
  while (*s != ',' || c != 0) {
    if (*s == '(' || *s == '[' || *s == '{')
      c++;
    if (*s == ')' || *s == ']' || *s == '}')
      c--;
    cerr << *s++;
  }
  cerr << "=" << head << ", ";
  _dump(s + 1, tail...);
}

#define dump(...)                                                              \
  do {                                                                         \
    fprintf(stderr, "%s:%d - ", __PRETTY_FUNCTION__, __LINE__);                \
    _dump(#__VA_ARGS__, __VA_ARGS__);                                          \
  } while (0)

template <typename Iter> ostream &_out(ostream &s, Iter b, Iter e) {
  s << "[";
  for (auto it = b; it != e; it++)
    s << (it == b ? "" : " ") << *it;
  s << "]";
  return s;
}

template <typename A, typename B>
ostream &operator<<(ostream &s, const pair<A, B> &p) {
  return s << "(" << p.first << "," << p.second << ")";
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &c) {
  return _out(s, ALL(c));
}
template <typename T, size_t N>
ostream &operator<<(ostream &s, const array<T, N> &c) {
  return _out(s, ALL(c));
}
template <typename T> ostream &operator<<(ostream &s, const set<T> &c) {
  return _out(s, ALL(c));
}
template <typename A, typename B>
ostream &operator<<(ostream &s, const map<A, B> &c) {
  return _out(s, ALL(c));
}
#else
#define dump(...)
#endif

template <typename T> void _R(T &x) { cin >> x; }
void _R(int &x) { scanf("%d", &x); }
void _R(long long &x) { scanf("%" PRId64, &x); }
void _R(double &x) { scanf("%lf", &x); }
void _R(char &x) { scanf(" %c", &x); }
void _R(char *x) { scanf("%s", x); }

void R() {}
template <typename T, typename... U> void R(T &head, U &...tail) {
  _R(head);
  R(tail...);
}

template <typename T> void _W(const T &x) { cout << x; }
void _W(const int &x) { printf("%d", x); }
template <typename T> void _W(const vector<T> &x) {
  for (auto i = x.cbegin(); i != x.cend(); i++) {
    if (i != x.cbegin())
      putchar(' ');
    _W(*i);
  }
}

void W() {}
template <typename T, typename... U> void W(const T &head, const U &...tail) {
  _W(head);
  putchar(sizeof...(tail) ? ' ' : '\n');
  W(tail...);
}

#ifdef SHIK
#define FILEIO(...)
#else
#define FILEIO(name)                                                           \
  do {                                                                         \
    freopen(name ".in", "r", stdin);                                           \
    freopen(name ".out", "w", stdout);                                         \
  } while (0)
#endif

// }}}

struct DJS {
  vector<int> fa, sz;
  void init(int n) {
    n++; // be nice for 1-index usage
    fa.resize(n);
    sz.resize(n);
    for (int i = 0; i < n; i++)
      sz[fa[i] = i] = 1;
  }
  int f(int x) { return x == fa[x] ? x : fa[x] = f(fa[x]); }
  void u(int a, int b) {
    a = f(a);
    b = f(b);
    if (a == b)
      return;
    if (sz[a] > sz[b])
      swap(a, b);
    fa[a] = b;
    sz[b] += sz[a];
  }
} djs;

bool cmp(int x, int y) { return MP(x % 2, x) < MP(y % 2, y); }

const int M = 110;
int n, m, a[M], bn, b[M];

bool chk() {
  djs.init(n);
  int s = 0;
  REP(i, m) {
    REP1(j, 1, a[i]) djs.u(s + j, s + a[i] - j + 1);
    s += a[i];
  }
  if (s != n)
    return 0;
  s = 0;
  REP(i, bn) {
    REP1(j, 1, b[i]) djs.u(s + j, s + b[i] - j + 1);
    s += b[i];
  }
  if (s != n)
    return 0;
  REP1(i, 1, n) if (djs.f(i) != djs.f(1)) return 0;
  return 1;
}

void output() {
  W(VI(a, a + m));
  W(bn);
  W(VI(b, b + bn));
  assert(chk());
}

int main() {
  R(n, m);
  REP(i, m) R(a[i]);
  sort(a, a + m, cmp);
  dump(VI(a, a + m));
  int odd = 0;
  REP(i, m) if (a[i] % 2 == 1) odd++;
  if (odd == 0) {
    REP(i, m) b[bn++] = a[i];
    b[0]--;
    b[bn++] = 1;
    output();
  } else if (odd == 1) {
    if (m == 1) {
      if (a[0] == 1) {
        b[bn++] = 1;
      } else {
        b[bn++] = a[0] - 1;
        b[bn++] = 1;
      }
    } else {
      REP(i, m) b[bn++] = a[i];
      b[0]--;
      b[bn - 1]++;
    }
    output();
  } else if (odd == 2) {
    if (m == 2) {
      if (n == 2) {
        b[bn++] = 2;
        output();
      } else {
        b[bn++] = n - 2;
        b[bn++] = 2;
        if (chk())
          output();
        else {
          swap(b[0], b[1]);
          if (chk())
            output();
        }
      }
    } else {
      swap(a[0], a[m - 2]);
      REP1(i, 1, m - 1) b[bn++] = a[i];
      b[0] += a[0] - 1;
      b[bn - 1]++;
      if (chk())
        output();
      else {
        b[0] -= 2;
        b[1] += 2;
        if (chk())
          output();
      }
    }
  } else {
    W("Impossible");
  }
  return 0;
}