B - Internship （网络流关键割边）

  1 #include<iostream>
  2 #include<stack>
  3 #include<queue>
  4 #include<iomanip>
  5 #include<stdio.h>
  6 #include<cstring>
  7 #include<cstring>
  8 #include<cmath>
  9 #include<algorithm>
 10 #include<map>
 11 #include<vector>
 12 using namespace std;
 13 # define ll long long
 14 const int maxn = 4e3+100;
 15 # define inf 0x3f3f3f3f
 16 int st,ed;
 17 int prev[maxn];
 18 int head[maxn];
 19 int line[maxn];
 20 int vis1[maxn],vis2[maxn];
 21 struct node
 22 {
 23     int fr;
 24     int to;
 25     int flow;
 26     int nex;
 27 } edge[maxn];
 28 int num;
 29 void init()
 30 {
 31     memset(head,-1,sizeof(head));
 32     memset(vis1,0,sizeof(vis1));
 33     memset(vis2,0,sizeof(vis2));
 34     num=0;
 35 }
 36 void addedge(int fr,int to,int flow)
 37 {
 38     edge[num].to=to;
 39     edge[num].fr=fr;
 40     edge[num].flow=flow;
 41     edge[num].nex=head[fr];
 42     head[fr]=num++;
 43     edge[num].to=fr;
 44     edge[num].fr=to;
 45     edge[num].flow=0;
 46     edge[num].nex=head[to];
 47     head[to]=num++;
 48 }
 49 bool bfs()
 50 {
 51     memset(prev,-1,sizeof(prev));
 52     prev[st]=1;
 53     queue<int>q;
 54     q.push(st);
 55     while(!q.empty())
 56     {
 57         int top=q.front();
 58         q.pop();
 59         for(int i=head[top]; i!=-1; i=edge[i].nex)
 60         {
 61             int temp=edge[i].to;
 62             if(prev[temp]==-1&&edge[i].flow>0)
 63             {
 64                 prev[temp]=prev[top]+1;
 65                 q.push(temp);
 66             }
 67         }
 68     }
 69     return prev[ed]!=-1;
 70 }
 71 int dfs(int u,int flow)
 72 {
 73     if(u==ed)
 74         return flow;
 75     int res=0;
 76     for(int i=head[u]; i!=-1; i=edge[i].nex)
 77     {
 78         int t=edge[i].to;
 79         if(prev[t]==(prev[u]+1)&&edge[i].flow>0)
 80         {
 81             int temp=dfs(t,min(flow,edge[i].flow));
 82             edge[i].flow-=temp;
 83             edge[i^1].flow+=temp;
 84             res+=temp;
 85             flow-=temp;
 86             if(flow==0)
 87                 break;
 88         }
 89     }
 90     if(res==0)
 91         prev[u]=-1;
 92     return res;
 93 }
 94 void  dinic()
 95 {
 96     int ans=0;
 97     while(bfs())
 98     {
 99         ans+=dfs(st,inf);
100     }
101 }
102 void dfs1(int u)
103 {
104     vis1[u]=1;
105     for(int i=head[u]; i!=-1; i=edge[i].nex)
106     {
107         int v=edge[i].to;
108         if(!vis1[v]&&edge[i].flow)
109             dfs1(v);
110     }
111 }
112 void dfs2(int u)
113 {
114     vis2[u]=1;
115     for(int i=head[u]; i!=-1; i=edge[i].nex)
116     {
117         int v=edge[i].to;
118         if(!vis2[v]&&edge[i^1].flow)
119         {
120             dfs2(v);
121         }
122     }
123 }
124 int main()
125 {
126     int n,m,l;
127     while(~scanf("%d",&n))
128     {
129         if(n==0)
130             break;
131         init();
132         scanf("%d %d",&m,&l);
133         st=n+m+1;
134         ed=0;
135         int t1,t2,t3;
136         for(int i=0; i<l; i++)
137         {
138             scanf("%d %d %d",&t1,&t2,&t3);
139             line[i]=num;
140             addedge(t1,t2,t3);
141         }
142         for(int i=1; i<=n; i++)
143         {
144             addedge(st,i,inf);
145         }
146         dinic();
147         dfs1(st);
148         dfs2(ed);
149        // cout<<1<<endl;
150         int flag=1;
151         for(int i=0; i<l; i++)
152         {
153       //  cout<<i<<endl;
154             if(edge[line[i]].flow==0&&vis1[edge[line[i]].fr]&&vis2[edge[line[i]].to])
155             {
156            //     cout<<i<<endl;
157                 if(flag)
158                 {
159                     printf("%d",i+1);
160                     flag=0;
161                 }
162                 else
163                 {
164                     printf(" %d",i+1);
165                 }
166             }
167         }
168         printf("\n");
169     }
170     return 0;
171 }