Problem p01680 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

#define GET_BIT(n, i) (((n) & (1 << ((i)-1))) >> ((i)-1)) // i start from 1
#define SET_BIT(n, i) ((n) | (1 << ((i)-1)))
#define CLR_BIT(n, i) ((n) & ~(1 << ((i)-1)))
#define SHOW_A(x)                                                              \
  { cout << #x << " = " << x << endl; }
#define SHOW_B(x, y)                                                           \
  { cout << #x << " = " << x << ", " << #y << " = " << y << endl; }
#define SHOW_C(x, y, z)                                                        \
  {                                                                            \
    cout << #x << " = " << x << ", " << #y << " = " << y << ", " << #z         \
         << " = " << z << endl;                                                \
  }
#define REACH_HERE                                                             \
  { cout << "REACH_HERE! line: " << __LINE__ << endl; }

const double E = 1e-8;
const double PI = acos(-1);

using namespace std;

const int MAX_N = 100005, MAX_M = 10005;
const int MOD_BASE = 1000000007;

int baba[MAX_N];

int power_modulo(int n, int p, int M) {
  int result = 1;
  while (p > 0) {
    if (p % 2 == 1)
      result = ((long long)result * n) % M;
    p /= 2;
    n = ((long long)n * n) % M;
  }
  return result;
}

int find_baba(int i) {
  if (baba[i] != i) {
    int new_baba = find_baba(baba[i]);
    baba[i] = new_baba;
    return new_baba;
  }
  return i;
}

void solve(int n, int m) {
  for (int i = 1; i <= n; i++) {
    baba[i] = i;
  }
  bool unstable = false;
  int total = 0;
  for (int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    if (baba[b] != b) {
      baba[baba[b]] = a;
    } else {
      baba[b] = a;
    }
    unstable = true;
  }
  int group = 0;
  for (int i = 1; i <= n; i++) {
    if (find_baba(i) == i) {
      group++;
    }
  }
  int ans = unstable ? 1 : 0;
  ans += power_modulo(2, group, MOD_BASE);
  cout << ans << endl;
}

int main() {
  ios::sync_with_stdio(false);

  while (true) {
    int n, m;
    cin >> n >> m;
    if (n == 0 && m == 0)
      break;
    solve(n, m);
  }

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>

#define GET_BIT(n, i) (((n) & (1 << ((i)-1))) >> ((i)-1)) // i start from 1
#define SET_BIT(n, i) ((n) | (1 << ((i)-1)))
#define CLR_BIT(n, i) ((n) & ~(1 << ((i)-1)))
#define SHOW_A(x)                                                              \
  { cout << #x << " = " << x << endl; }
#define SHOW_B(x, y)                                                           \
  { cout << #x << " = " << x << ", " << #y << " = " << y << endl; }
#define SHOW_C(x, y, z)                                                        \
  {                                                                            \
    cout << #x << " = " << x << ", " << #y << " = " << y << ", " << #z         \
         << " = " << z << endl;                                                \
  }
#define REACH_HERE                                                             \
  { cout << "REACH_HERE! line: " << __LINE__ << endl; }

const double E = 1e-8;
const double PI = acos(-1);

using namespace std;

const int MAX_N = 100005, MAX_M = 10005;
const int MOD_BASE = 1000000007;

int baba[MAX_N];

int power_modulo(int n, int p, int M) {
  int result = 1;
  while (p > 0) {
    if (p % 2 == 1)
      result = ((long long)result * n) % M;
    p /= 2;
    n = ((long long)n * n) % M;
  }
  return result;
}

int find_baba(int i) {
  if (baba[i] != i) {
    int new_baba = find_baba(baba[i]);
    baba[i] = new_baba;
    return new_baba;
  }
  return i;
}

void solve(int n, int m) {
  for (int i = 1; i <= n; i++) {
    baba[i] = i;
  }
  bool unstable = false;
  int total = 0;
  for (int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    baba[find_baba(b)] = find_baba(a);
    unstable = true;
  }
  int group = 0;
  for (int i = 1; i <= n; i++) {
    if (find_baba(i) == i) {
      group++;
    }
  }
  int ans = unstable ? 1 : 0;
  ans += power_modulo(2, group, MOD_BASE);
  cout << ans << endl;
}

int main() {
  ios::sync_with_stdio(false);

  while (true) {
    int n, m;
    cin >> n >> m;
    if (n == 0 && m == 0)
      break;
    solve(n, m);
  }

  return 0;
}