Problem p03321 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ul;
typedef unsigned int ui;
typedef long double ld;
const ll mod = 1000000007;
const ll INF = mod * mod;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<int, int> P;
typedef pair<ld, ld> LDP;
typedef pair<ll, ll> LP;
typedef vector<int> vec;
typedef vector<string> svec;
#define fr first
#define sc second
#define all(c) c.begin(), c.end()
#define pb push_back
// #define int long long

// 補グラフをとるとクリークと独立集合は互いに移りあう

int n, m;
int G[2000][2000];

bool isbipartite = true;
int color[2000];
int colornum[2];

void dfs(int v, int c) {
  color[v] = c;
  colornum[c]++;
  rep(i, n) {
    if (i == v || !G[v][i])
      continue;
    if (color[i] = -1)
      dfs(i, 1 - c);
    else if (color[i] == c)
      isbipartite = false;
  }
}

void solve() {
  cin >> n >> m;
  rep(i, n) {
    rep(j, n) { G[i][j] = 1; }
  }
  rep(i, m) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    G[a][b] = G[b][a] = 0;
  }
  vector<P> nums;
  rep(i, n) color[i] = -1;
  rep(v, n) {
    if (color[v] != -1)
      continue;
    colornum[0] = colornum[1] = 0;
    dfs(v, 0);
    nums.pb(P(colornum[0], colornum[1]));
  }

  if (!isbipartite) {
    cout << -1 << endl;
    return;
  }

  bool dp[2000];
  rep(i, n) dp[i] = 0;
  dp[0] = 1;
  for (auto p : nums) {
    for (int j = n; j >= 0; j--) {
      if (!dp[j])
        continue;
      dp[j] = 0;
      dp[j + p.fr] = 1;
      dp[j + p.sc] = 1;
    }
  }
  int a = -1;
  int mid = n;
  rep(i, n + 1) {
    if (!dp[i])
      continue;
    if (mid > abs(i - n / 2))
      mid = abs(i - n / 2), a = i;
  }
  int b = n - a;
  cout << a * (a - 1) / 2 + b * (b - 1) / 2 << endl;
}

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  // cout << fixed << setprecision(10);
  // init();
  solve();
  // cout << "finish" << endl;
  return 0;
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ul;
typedef unsigned int ui;
typedef long double ld;
const ll mod = 1000000007;
const ll INF = mod * mod;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<int, int> P;
typedef pair<ld, ld> LDP;
typedef pair<ll, ll> LP;
typedef vector<int> vec;
typedef vector<string> svec;
#define fr first
#define sc second
#define all(c) c.begin(), c.end()
#define pb push_back
// #define int long long

// 補グラフをとるとクリークと独立集合は互いに移りあう

int n, m;
int G[2000][2000];

bool isbipartite = true;
int color[2000];
int colornum[2];

void dfs(int v, int c) {
  color[v] = c;
  colornum[c]++;
  rep(i, n) {
    if (i == v || !G[v][i])
      continue;
    if (color[i] == -1)
      dfs(i, 1 - c);
    else if (color[i] == c)
      isbipartite = false;
  }
}

void solve() {
  cin >> n >> m;
  rep(i, n) {
    rep(j, n) { G[i][j] = 1; }
  }
  rep(i, m) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    G[a][b] = G[b][a] = 0;
  }
  vector<P> nums;
  rep(i, n) color[i] = -1;
  rep(v, n) {
    if (color[v] != -1)
      continue;
    colornum[0] = colornum[1] = 0;
    dfs(v, 0);
    nums.pb(P(colornum[0], colornum[1]));
  }

  if (!isbipartite) {
    cout << -1 << endl;
    return;
  }

  bool dp[2000];
  rep(i, n) dp[i] = 0;
  dp[0] = 1;
  for (auto p : nums) {
    for (int j = n; j >= 0; j--) {
      if (!dp[j])
        continue;
      dp[j] = 0;
      dp[j + p.fr] = 1;
      dp[j + p.sc] = 1;
    }
  }
  int a = -1;
  int mid = n;
  rep(i, n + 1) {
    if (!dp[i])
      continue;
    if (mid > abs(i - n / 2))
      mid = abs(i - n / 2), a = i;
  }
  int b = n - a;
  cout << a * (a - 1) / 2 + b * (b - 1) / 2 << endl;
}

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  // cout << fixed << setprecision(10);
  // init();
  solve();
  // cout << "finish" << endl;
  return 0;
}