UVA-10462.Is There A Second Way Left(Kruskal+次小生成树)

题目链接

本题大意：这道题用Kruskal较为容易

参考代码：

 #include <cstdio>
 #include <cstring>
 #include <vector>
 #include <algorithm>
 using namespace std;

 const int maxn =  + , maxe =  + , INF = 0x3f3f3f3f;
 int n, m, pir[maxn], Max[maxn][maxn];
 vector <int> G[maxn];
 struct Edge {
     int u, v, w;
     bool vis;
 }edge[maxe];

 int Find(int x) {
     if(x == pir[x]) return x;
     return pir[x] = Find(pir[x]);
 }

 bool cmp(const Edge &a, const Edge &b) {
     return a.w < b.w;
 }

 int Kruskal() {
     sort(edge + , edge + m + , cmp);
     int ans = , cnt = ;
     for(int i = ; i <= n; i ++) {
         G[i].clear();
         G[i].push_back(i);
         pir[i] = i;
     }
     for(int i = ; i <= m; i ++) {
         int fx = Find(edge[i].u), fy = Find(edge[i].v);
         if(cnt == n - ) break;
         if(fx != fy) {
             cnt ++;
             edge[i].vis = true;
             ans += edge[i].w;
             int len_fx = G[fx].size(), len_fy = G[fy].size();
             for(int j = ; j < len_fx; j ++) {
                 for(int k = ; k < len_fy; k ++) {
                     Max[G[fx][j]][G[fy][k]] = Max[G[fy][k]][G[fx][j]] = edge[i].w;
                 }
             }
             pir[fx] = fy;
             for(int j = ; j < len_fx; j ++)
                 G[fy].push_back(G[fx][j]);
         }
     }
     if(cnt < n - ) return INF;
     return ans;
 }

 int Second_Kruskal(int MST) {
     int ans = INF;
     for(int i = ; i <= m; i ++) {
         if(!edge[i].vis)
             ans = min(ans, MST + edge[i].w - Max[edge[i].u][edge[i].v]);
     }
     return ans;
 }

 int main () {
     int t, Case = ;
     scanf("%d", &t);
     while(t --) {
         scanf("%d %d", &n, &m);
         for(int i = ; i <= m; i ++) {
             scanf("%d %d %d", &edge[i].u, &edge[i].v, &edge[i].w);
             edge[i].vis = false;
         }
         printf("Case #%d : ", ++Case);
         int MST = Kruskal();
         if(MST == INF) {
             printf("No way\n");
             continue;
         }
         int Second_MST = Second_Kruskal(MST);
         if(Second_MST == INF) printf("No second way\n");
         else printf("%d\n", Second_MST);
     }
     return ;
 }