Problem p01279 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <cstdio>
#include <vector>
using namespace std;

bool match(int x, const vector<vector<int>> &xg, vector<bool> &visited,
           vector<int> &p) {
  if (x < 0)
    return true;
  if (visited[x])
    return false;
  visited[x] = true;

  for (int i = 0; i < xg[x].size(); ++i) {
    int y = xg[x][i];
    if (match(p[y], xg, visited, p)) {
      p[y] = x;
      return true;
    }
  }
  return false;
}

int max_matching(const vector<vector<int>> &xg, const int ys) {
  vector<int> p(ys, -1);
  vector<bool> visited(xg.size());
  int count = 0;
  for (int x = 0; x < xg.size(); ++x) {
    fill(visited.begin(), visited.end(), false);
    if (match(x, xg, visited, p))
      count += 1;
  }
  return count;
}

int main() {
  while (true) {

    int n, m;
    vector<pair<int, pair<int, int>>> troop;
    vector<pair<int, int>> base;

    scanf("%d%d", &n, &m);
    if (n == 0)
      break;
    troop.resize(n);
    base.resize(m);
    for (int i = 0; i < n; ++i) {
      int t1, t2, t3;
      scanf("%d%d%d", &t1, &t2, &t3);
      troop[i] = make_pair(t3, make_pair(t1, t2));
    }
    for (int i = 0; i < m; ++i) {
      int t1, t2;
      scanf("%d%d", &t1, &t2);
      base[i] = make_pair(t1, t2);
    }

    double lb = 0, ub = 1e6;

    for (int step = 0; step < 10000; ++step) {

      double mid = (lb + ub) / 2;

      vector<vector<int>> xg(n);

      for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {

          int dx = troop[i].second.first - base[j].first;
          int dy = troop[i].second.second - base[j].second;
          int speed = troop[i].first;
          int d2 = dx * dx + dy * dy;

          if (d2 <= speed * speed * mid * mid)
            xg[i].push_back(j);
        }
      }

      if (max_matching(xg, m) == m)
        ub = mid;
      else
        lb = mid;
    }

    double ans = lb;

    printf("%.8lf\n", ans);
  }

  return 0;
}

Fixed submission:
#include <cstdio>
#include <vector>
using namespace std;

bool match(int x, const vector<vector<int>> &xg, vector<bool> &visited,
           vector<int> &p) {
  if (x < 0)
    return true;
  if (visited[x])
    return false;
  visited[x] = true;

  for (int i = 0; i < xg[x].size(); ++i) {
    int y = xg[x][i];
    if (match(p[y], xg, visited, p)) {
      p[y] = x;
      return true;
    }
  }
  return false;
}

int max_matching(const vector<vector<int>> &xg, const int ys) {
  vector<int> p(ys, -1);
  vector<bool> visited(xg.size());
  int count = 0;
  for (int x = 0; x < xg.size(); ++x) {
    fill(visited.begin(), visited.end(), false);
    if (match(x, xg, visited, p))
      count += 1;
  }
  return count;
}

int main() {
  while (true) {

    int n, m;
    vector<pair<int, pair<int, int>>> troop;
    vector<pair<int, int>> base;

    scanf("%d%d", &n, &m);
    if (n == 0)
      break;
    troop.resize(n);
    base.resize(m);
    for (int i = 0; i < n; ++i) {
      int t1, t2, t3;
      scanf("%d%d%d", &t1, &t2, &t3);
      troop[i] = make_pair(t3, make_pair(t1, t2));
    }
    for (int i = 0; i < m; ++i) {
      int t1, t2;
      scanf("%d%d", &t1, &t2);
      base[i] = make_pair(t1, t2);
    }

    double lb = 0, ub = 1e6;

    for (int step = 0; step < 100; ++step) {

      double mid = (lb + ub) / 2;

      vector<vector<int>> xg(n);

      for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {

          int dx = troop[i].second.first - base[j].first;
          int dy = troop[i].second.second - base[j].second;
          int speed = troop[i].first;
          int d2 = dx * dx + dy * dy;

          if (d2 <= speed * speed * mid * mid)
            xg[i].push_back(j);
        }
      }

      if (max_matching(xg, m) == m)
        ub = mid;
      else
        lb = mid;
    }

    double ans = lb;

    printf("%.8lf\n", ans);
  }

  return 0;
}