Problem p01126 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <algorithm>
#include <cassert>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>

using namespace std;

#define LOG(...) printf(__VA_ARGS__)
// #define LOG(...)
#define FOR(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define REP(i, n) for (int i = 0; i < (int)(n); ++i)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
#define RSORT(c) sort((c).rbegin(), (c).rend())
#define CLR(a) memset((a), 0, sizeof(a))

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, 1, 0, -1};

int main() {
  int v_n, h_n, start;
  while (cin >> v_n >> h_n >> start, v_n | h_n | start) {
    vector<vector<pii>> lines(v_n + 1);
    REP(i, h_n) {
      int h, p, q;
      cin >> h >> p >> q;
      lines[p].push_back({h, q});
      lines[q].push_back({h, p});
    }

    REP(i, v_n + 1) { RSORT(lines[i]); }

    int now_h = 10000;
    int now_v = start;
    while (1) {
      bool update = false;
      for (auto h_line : lines[now_v]) {
        if (now_h > h_line.first) {
          //                    LOG("move to %d -> %d\\n", now_v, h_line.second);
          now_h = h_line.first;
          now_v = h_line.second;
          update = true;
          break;
        }
      }
      if (!update)
        break;
    }

    cout << now_v << endl;
  }
}

Error: TLE

Fixed submission:

#include <algorithm>
#include <cassert>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>

using namespace std;

#define LOG(...) printf(__VA_ARGS__)
// #define LOG(...)
#define FOR(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define REP(i, n) for (int i = 0; i < (int)(n); ++i)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
#define RSORT(c) sort((c).rbegin(), (c).rend())
#define CLR(a) memset((a), 0, sizeof(a))

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, 1, 0, -1};

int main() {
  int v_n, h_n, start;
  while (cin >> v_n >> h_n >> start, v_n | h_n | start) {
    vector<vector<pii>> lines(v_n + 1);
    REP(i, h_n) {
      int h, p, q;
      cin >> h >> p >> q;
      lines[p].push_back({h, q});
      lines[q].push_back({h, p});
    }

    int now_h = 10000;
    int now_v = start;
    while (1) {
      bool update = false;
      for (auto h_line : lines[now_v]) {
        if (now_h >= h_line.first) {
          //                    LOG("move to %d -> %d\\n", now_v, h_line.second);
          now_h = h_line.first;
          now_v = h_line.second;
          update = true;
          break;
        }
      }
      if (!update)
        break;
    }

    cout << now_v << endl;
  }
}