【luogu P2764 最小路径覆盖问题】 模板

题目链接：https://www.luogu.org/problemnew/show/P2764

把每个点在左边建一遍右边建一遍，再加上源点汇点，跑最大流，n-最大流就是答案。

 #include <iostream>
 #include <cstdio>
 #include <algorithm>
 #include <queue>
 #include <cstring>
 using namespace std;
 const int inf = 1e9;
 const int maxn = ;
 int n, m, s, t, deep[maxn], maxflow, pre[maxn], st;
 struct EDG{
     int next, to, flow;
 }edge[maxn];
 int cnt = -, head[maxn], cur[maxn];
 queue<int> q; void add(int u, int v, int w)
 {
     edge[++cnt].next = head[u];
     edge[cnt].to = v;
     edge[cnt].flow = w;
     head[u] = cnt;     edge[++cnt].next = head[v];
     edge[cnt].to = u;
     edge[cnt].flow = ;
     head[v] = cnt;
 } bool bfs(int s, int t)
 {
     for(int i = s; i <= t; i++)
     {
         cur[i] = head[i];
         deep[i] = ;
     }
     deep[s] = ;
     queue<int> q;
     q.push(s);
     while(!q.empty())
     {
         int now = q.front(); q.pop();
         for(int i = head[now]; i != -; i = edge[i].next)
         {
             if(!deep[edge[i].to] && edge[i].flow)
             {
                 q.push(edge[i].to);
                 deep[edge[i].to] = deep[now]+;
             }
         }
     }
     if(deep[t]) return true;
     return false;
 }
 int dfs(int now, int t, int limit)
 {
     if(!limit || now == t) return limit;
     int flow = , f;
     for(int i = cur[now]; i != -; i = edge[i].next)
     {
         cur[now] = i;
         if(deep[edge[i].to] == deep[now]+ && (f = dfs(edge[i].to, t, min(limit, edge[i].flow))))
         {
             flow += f;
             limit -= f;
             edge[i].flow -= f;
             edge[i^].flow += f;
             if(!limit) break;
         }
     }
     return flow;
 }
 void Dinic(int s, int t)
 {
     while(bfs(s,t))
     maxflow += dfs(s,t,inf);
 }
 void output(int x)
 {
     printf("%d ",x);
     for(int i = head[x]; i != -; i = edge[i].next)
     {
         if(edge[i].flow ==  && edge[i].to - n >  && edge[i].to - n <= n)
         {
             output(edge[i].to - n);
             break;
         }
     }
     return ;
 }
 int main()
 {
     scanf("%d%d",&n,&m);
     for(int i = ; i <= *n+; i++)
     head[i] = -;
     s = , t = *n+;
     for(int i = ; i <= m; i++)
     {
         int u, v;
         scanf("%d%d",&u,&v);
         add(u,v+n,);
     }
     for(int i = ; i <= n; i++)
     {
         add(,i,);
         add(i+n,*n+,);
     }
     Dinic(s,t);
     for(int i=head[*n+];i!=-;i=edge[i].next)
     {
         if(edge[i^].flow==)
             pre[++st]=edge[i].to;
     }
     for(int i=;i<=st;i++)
     {
         output(pre[i]-n);
         printf("\n");
     }
     printf("%d",n - maxflow);
     return ;
 }