HNU 13375 Flowery Trails (spfa最短路)

 1 #include <cstring>
 2 #include <cmath>
 3 #include <queue>
 4 #include <vector>
 5 #include <cstdio>
 6 #include <algorithm>
 7 using namespace std;
 8 typedef long long ll;
 9 const int maxn = 10005;
10 const int maxm = 600000;
11 const int INF = 0x3f3f3f3f;
12 int n, m;
13 struct ee {
14     int to;
15     int nxt;
16     int w;
17 } edge[maxm];
18 
19 int head[maxn], tol;
20 void init() {
21     memset(head, -1, sizeof head );
22     tol = 0;
23 }
24 void add(int u, int v, int w) {
25     edge[tol].to = v;
26     edge[tol].w = w;
27     edge[tol].nxt = head[u];
28     head[u] = tol++;
29 }
30 int d1[maxn], d2[maxn];
31 bool vis[maxn];
32 void spfa(int s, int t, int d[]) {
33     for(int i=0; i<n; ++i) d[i] = INF;
34     memset(vis, false, sizeof vis );
35     queue<int> q;
36     q.push(s);
37     d[s] = 0;
38     vis[s] = true;
39     while(!q.empty()) {
40         int u = q.front();
41         q.pop();
42         vis[u] = false;
43         for(int i=head[u]; ~i; i=edge[i].nxt) {
44             int &v = edge[i].to;
45             int &cost = edge[i].w;
46             if(d[v] > d[u] + cost) {
47                 d[v] = d[u] + cost;
48                 if(!vis[v]) {
49                     vis[v] = true;
50                     q.push(v);
51                 }
52             }
53         }
54     }
55 }
56 void solve() {
57     int s = 0, t = n-1;
58     spfa(s, t, d1);
59     spfa(t, s, d2);
60     ll ans = 0;
61     int minn = d1[t];
62     for(int u=0; u<n; u++) {
63         for(int i=head[u]; ~i; i=edge[i].nxt) {
64             int &v=edge[i].to;
65             int &cost=edge[i].w;
66             if(d1[u]+d2[v]+cost==minn) {
67                 ans+=cost;
68             }
69         }
70     }
71     printf("%I64d\n", ans*2);
72 }
73 int main() {
74     int u, v, l;
75     while(~scanf("%d%d", &n, &m)) {
76         init();
77         for(int i=0; i<m; ++i) {
78             scanf("%d%d%d", &u, &v, &l);
79             add(u, v, l);
80             add(v, u, l);
81         }
82         solve();
83     }
84     return 0;
85 }