最短路 Codeforces Round #103 (Div. 2) D. Missile Silos

题目传送门

 /*
     最短路： 不仅扫描边，还要扫描点；点有两种情况，一种刚好在中点，即从u，v都一样，那么最后/2
                 还有一种是从u，v不一样，两种的距离都是l
     模板错了，逗了好久:(
 */
 #include <cstdio>
 #include <algorithm>
 #include <cstring>
 #include <cmath>
 #include <vector>
 #include <queue>
 using namespace std;

 const int MAXN = 1e5 + ;
 const int INF = 0x3f3f3f3f;
 int d[MAXN];
 int cnt[MAXN];
 bool vis[MAXN];
 vector<pair<int, int> > G[MAXN];
 int n, m, s, l, ans, ans2;

 void SPFA(void) {
     memset (vis, false, sizeof (vis));
     memset (d, INF, sizeof (d));  d[s] = ;
     queue<int> Q;   Q.push (s);
     while (!Q.empty ()) {
         int u = Q.front (); Q.pop ();
         vis[u] = false;
         for (int i=; i<G[u].size (); ++i)  {
             int v = G[u][i].first;    int w = G[u][i].second;
             if (d[v] > d[u] + w)    {
                 d[v] = d[u] + w;
                 if (!vis[v])    {
                     vis[v] = true;  Q.push (v);
                 }
             }
         }
     }
 }

 int main(void)  {       //Codeforces Round #103 (Div. 2) D. Missile Silos
     //freopen ("spfa.in", "r", stdin);

     while (scanf ("%d%d%d", &n, &m, &s) == )   {
         for (int i=; i<=m; ++i)    {
             int u, v, w;    scanf ("%d%d%d", &u, &v, &w);
             G[u].push_back (make_pair (v, w)); G[v].push_back (make_pair (u, w));
         }
         scanf ("%d", &l);   SPFA ();

         ans = ans2 = ;
         for (int i=; i<=n; ++i)    {
             for (int j=; j<G[i].size (); ++j)  {
                 int u = i, v = G[i][j].first, w = G[i][j].second;
                 if (d[u] < l && l - d[u] < w)   {
                     if (w - (l-d[u]) + d[v] > l)    ans++;
                     else if (d[u] + d[v] + w ==  * l)  ans2++;
                 }
             }
             if (d[i] == l)  ans++;
         }
         printf ("%d\n", ans + ans2 / );
     }

     return ;
 }