URAL 1416 Confidential（次小生成树）

 1 #include <cstdio>
 2 #include <algorithm>
 3 #include <cstring>
 4 #include <iostream>
 5 using namespace std;
 6 
 7 const int MAXV = 510;
 8 const int MAXE = MAXV * MAXV;
 9 const int INF = 0x3f3f3f3f;
10 
11 int head[MAXV], ecnt;
12 int to[MAXE], next[MAXE], cost[MAXE];
13 bool select[MAXE];
14 int n, m;
15 
16 void init() {
17     memset(head + 1, -1, n * sizeof(int));
18     ecnt = 0;
19 }
20 
21 void add_edge(int u, int v, int c) {
22     to[ecnt] = v; cost[ecnt] = c; next[ecnt] = head[u]; head[u] = ecnt++;
23     to[ecnt] = u; cost[ecnt] = c; next[ecnt] = head[v]; head[v] = ecnt++;
24 }
25 
26 int dis[MAXV], pre[MAXV];
27 int maxPath[MAXV][MAXV];
28 bool vis[MAXV];
29 
30 int prim() {
31     memset(dis + 1, 0x3f, n * sizeof(int));
32     dis[1] = 0;
33     int res = 0;
34     for(int _ = 0; _ < n; ++_) {
35         int u = -1;
36         for(int i = 1; i <= n; ++i) if(!vis[i] && dis[i] < INF)
37             if(u == -1 || dis[i] < dis[u]) u = i;
38         if(u == -1) return -1;
39         for(int p = head[u]; ~p; p = next[p]) {
40             int &v = to[p];
41             if(vis[v]) continue;
42             if(cost[p] < dis[v]) dis[v] = cost[p], pre[v] = p;
43         }
44         for(int i = 1; i <= n; ++i) if(vis[i]) {
45             int &v = to[pre[u] ^ 1];
46             maxPath[i][u] = maxPath[u][i] = max(maxPath[i][v], dis[u]);
47         }
48         res += dis[u];
49         vis[u] = true;
50         if(u != 1) select[pre[u]] = select[pre[u] ^ 1] = true;
51     }
52     return res;
53 }
54 
55 int solve(int ans) {
56     int res = -1;
57     for(int u = 1; u <= n; ++u) {
58         for(int p = head[u]; ~p; p = next[p]) {
59             int &v = to[p];
60             if(select[p] || u == v) continue;
61             if(res == -1 || ans - maxPath[u][v] + cost[p] < res)
62                 res = ans - maxPath[u][v] + cost[p];
63         }
64     }
65     return res;
66 }
67 
68 int main() {
69     scanf("%d%d", &n, &m);
70     init();
71     for(int i = 0, a, b, c; i < m; ++i) {
72         scanf("%d%d%d", &a, &b, &c);
73         add_edge(a, b, c);
74     }
75     int ans1 = prim(), ans2 = -1;
76     if(ans1 != -1) ans2 = solve(ans1);
77     printf("Cost: %d\n", ans1);
78     printf("Cost: %d\n", ans2);
79 }