Problem p00750 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <utility>
#include <vector>

using namespace std;

#define rep(i, n) for ((i) = 0; (i) < (int)(n); (i)++)
#define foreach(itr, c)                                                        \
  for (__typeof((c).begin()) itr = (c).begin(); itr != (c).end(); itr++)
template <class T> void pp(T a, T b) {
  for (T i = a; i != b; ++i)
    cout << *i << ' ';
  cout << endl;
}

#define INF "|"

struct Edge {
  int from, to;
  string cost;
  Edge(int from_, int to_, string cost_) {
    from = from_;
    to = to_;
    cost = cost_;
  }
  Edge() {
    from = to = 0;
    cost = "";
  }
};

int n, a, s, g;
Edge edge[410];
string d[50];

int main() {
  int i;

  while (1) {
    scanf("%d %d %d %d", &n, &a, &s, &g);
    if (n == 0 && a == 0 && s == 0 && g == 0)
      break;

    rep(i, a) {
      int from, to;
      string cost;
      // cin >> from >> to >> cost;
      cin >> to >> from >> cost;
      Edge e(from, to, cost);
      edge[i] = e;
    }

    rep(i, n) d[i] = INF;
    d[g] = "";
    int count = 0, cc = 0;
    while (1) {
      bool update = false;
      rep(i, a) {
        Edge e = edge[i];
        if (d[e.from] != INF &&
            d[e.to] > e.cost + d[e.from]) { // d[0] > d[0] + "a"
          // cout << d[e.to]<< ' ' <<  d[e.from] << ' ' <<  e.cost << endl;
          d[e.to] = e.cost + d[e.from];
          update = true;
          if (e.to == s && count >= n + 1)
            cc++;
        }
      }
      count++;
      if (cc >= 1)
        break;
      if (count >= 1000)
        break;
      if (!update)
        break;
    }

    cout << ((d[s] == "|" || cc >= 1) ? "NO" : d[s]) << endl;
  }

  return 0;
}

Fixed submission:
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <utility>
#include <vector>

using namespace std;

#define rep(i, n) for ((i) = 0; (i) < (int)(n); (i)++)
#define foreach(itr, c)                                                        \
  for (__typeof((c).begin()) itr = (c).begin(); itr != (c).end(); itr++)
template <class T> void pp(T a, T b) {
  for (T i = a; i != b; ++i)
    cout << *i << ' ';
  cout << endl;
}

#define INF "|"

struct Edge {
  int from, to;
  string cost;
  Edge(int from_, int to_, string cost_) {
    from = from_;
    to = to_;
    cost = cost_;
  }
  Edge() {
    from = to = 0;
    cost = "";
  }
};

int n, a, s, g;
Edge edge[410];
string d[50];

int main() {
  int i;

  while (1) {
    scanf("%d %d %d %d", &n, &a, &s, &g);
    if (n == 0 && a == 0 && s == 0 && g == 0)
      break;

    rep(i, a) {
      int from, to;
      string cost;
      // cin >> from >> to >> cost;
      cin >> to >> from >> cost;
      Edge e(from, to, cost);
      edge[i] = e;
    }

    rep(i, n) d[i] = INF;
    d[g] = "";
    int count = 0, cc = 0;
    while (1) {
      bool update = false;
      rep(i, a) {
        Edge e = edge[i];
        if (d[e.from] != INF &&
            d[e.to] > e.cost + d[e.from]) { // d[0] > d[0] + "a"
          // cout << d[e.to]<< ' ' <<  d[e.from] << ' ' <<  e.cost << endl;
          d[e.to] = e.cost + d[e.from];
          update = true;
          if (e.to == s && count >= n + 1)
            cc++;
        }
      }
      count++;
      if (cc >= 1)
        break;
      if (count >= 500)
        break;
      if (!update)
        break;
    }

    cout << ((d[s] == "|" || cc >= 1) ? "NO" : d[s]) << endl;
  }

  return 0;
}