题目传送门
1 /*
2 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
3 还有一种是从u，v不一样，两种的距离都是l
4 模板错了，逗了好久:(
5 */
6 #include <cstdio>
7 #include <algorithm>
8 #include <cstring>
9 #include <cmath>
10 #include <vector>
11 #include <queue>
12 using namespace std;
13
14 const int MAXN = 1e5 + 10;
15 const int INF = 0x3f3f3f3f;
16 int d[MAXN];
17 int cnt[MAXN];
18 bool vis[MAXN];
19 vector<pair<int, int> > G[MAXN];
20 int n, m, s, l, ans, ans2;
21
22 void SPFA(void) {
23 memset (vis, false, sizeof (vis));
24 memset (d, INF, sizeof (d)); d[s] = 0;
25 queue<int> Q; Q.push (s);
26 while (!Q.empty ()) {
27 int u = Q.front (); Q.pop ();
28 vis[u] = false;
29 for (int i=0; i<G[u].size (); ++i) {
30 int v = G[u][i].first; int w = G[u][i].second;
31 if (d[v] > d[u] + w) {
32 d[v] = d[u] + w;
33 if (!vis[v]) {
34 vis[v] = true; Q.push (v);
35 }
36 }
37 }
38 }
39 }
40
41 int main(void) { //Codeforces Round #103 (Div. 2) D. Missile Silos
42 //freopen ("spfa.in", "r", stdin);
43
44 while (scanf ("%d%d%d", &n, &m, &s) == 3) {
45 for (int i=1; i<=m; ++i) {
46 int u, v, w; scanf ("%d%d%d", &u, &v, &w);
47 G[u].push_back (make_pair (v, w)); G[v].push_back (make_pair (u, w));
48 }
49 scanf ("%d", &l); SPFA ();
50
51 ans = ans2 = 0;
52 for (int i=1; i<=n; ++i) {
53 for (int j=0; j<G[i].size (); ++j) {
54 int u = i, v = G[i][j].first, w = G[i][j].second;
55 if (d[u] < l && l - d[u] < w) {
56 if (w - (l-d[u]) + d[v] > l) ans++;
57 else if (d[u] + d[v] + w == 2 * l) ans2++;
58 }
59 }
60 if (d[i] == l) ans++;
61 }
62 printf ("%d
", ans + ans2 / 2);
63 }
64
65 return 0;
66 }