hdu 4738 Caocao's Bridges(2013杭州网络赛丶神坑)

 

hdu 4738 Caocao's Bridges(2013杭州网络赛丶神坑)

分类： 图论 2013-09-15 19:32  84人阅读  评论(2)  收藏  举报

就是求最小权值的桥。。不过有好几个坑。。。

1：原图不连通，ans=0.

2： m<=n^2 显然有重边，重边必然不是桥，处理重边直接add(u, v, INF).

3:   最小桥边权为0的时候，ans=1，至少要派一个人运炸弹。。。

 #include<iostream>
 #include<algorithm>
 #include<vector>
 #include<string>
 #include<queue>
 #include<stack>
 #include<cstdio>
 #include<cstring>
 #include<cstdlib>
 #include<cmath>
 #include<fstream>
 #include<sstream>
 #include<map>
 #include<set>
 using namespace std;

 const int N = 1111;
 const int INF = 100000000;
 int n, m;
 int pre[N], low[N], dfs_clock;
 int bccno[N], vis[N], wi[N][N], g[N][N];
 struct Edge
 {
     //flag = 1 ->bridge
     int from, to, w, flag;
 };
 vector<int> G[N];
 vector<Edge> edges;

 //add bidir edge
 void addedge(int u, int v, int w)
 {
     edges.push_back((Edge){u, v, w, 0});
     edges.push_back((Edge){v, u, w, 0});
     int nima = edges.size();
     G[u].push_back(nima-2);
     G[v].push_back(nima-1);
 }

 int dfs(int u, int fa)
 {
     int lowu = pre[u] = ++dfs_clock;
     int sz = G[u].size();
     for(int i=0; i<sz; i++)
     {
         int v = edges[G[u][i]].to;
         if(!pre[v])
         {
             int lowv = dfs(v, u);
             lowu = min(lowu, lowv);
             if(lowv > pre[u]) edges[G[u][i]].flag = 1, edges[G[u][i]^1].flag = 1;
         }
         else if(pre[v] < pre[u] && v != fa) lowu = min(lowu, pre[v]);
     }
     return low[u] = lowu;
 }

 void dfs1(int u)
 {
     vis[u] = 1;
     int sz = G[u].size();
     for(int i=0; i<sz; i++)
     {
         Edge e = edges[G[u][i]];
         int v = e.to;
         if(!vis[v]) dfs1(v);
     }
 }
 int u, v, w;

 int main()
 {
     while(scanf("%d%d", &n, &m), n+m)
     {
         for(int i=0; i<n+1; i++) G[i].clear();
         edges.clear();
         memset(g, 0, sizeof(g));
         for(int i=0; i<m; i++)
         {
             scanf("%d%d%d", &u, &v, &w); u--; v--;
             g[u][v]++; //BIANSHU
             g[v][u]++;
             wi[u][v] = w;
         }

         //CHONG BIAN
         for(int i=0; i<n; i++)
             for(int j=0; j<n; j++)
             {
                 if(g[i][j] == 1) addedge(i, j, wi[i][j]);
                 else if(g[i][j] > 1) addedge(i, j, INF);
             }

         //BU LIANTONG
         memset(vis, 0, sizeof(vis));
         dfs1(0);
         bool flag = 0;
         for(int i=0; i<n; i++)
         {
             if(!vis[i])
             {
                 flag = 1;
                 break;
             }
         }
         if(flag)
         {
             puts("0");
             continue;
         }
         //QIAO
         memset(pre, 0, sizeof(pre));
         dfs_clock  = 0;
         for(int i=0; i<n; i++) if(!pre[i]) dfs(i, -1);
         int ans = INF;
         int sz = edges.size();
         for(int i=0; i<sz; i++) if(edges[i].flag == 1) ans = min(ans, edges[i].w);
         if(ans == INF) puts("-1");
         else printf("%d\n", ans == 0 ? 1 : ans); //0 -> 1
     }
     return 0;
 }