【loj】#6002. 「网络流 24 题」最小路径覆盖

//#define debug
#include <bits/stdc++.h>
using namespace std;
#define ALL(v) (v).begin(),(v).end()
#define cl(a,b) memset(a,b,sizeof(a))
#define clr clear()
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define fin freopen("D://in.txt","r",stdin)
#define fout freopen("D://out.txt","w",stdout)
#define lson t<<1,l,mid
#define rson t<<1|1,mid+1,r
#define seglen(t) (node[t].r-node[t].l+1)
#define lowbit(x) (x)&(-x)
typedef long long LL;
typedef pair<int,int> PII;
const int INF = 0x3f3f3f3f;
//O(N^2*M)
const int MAX_V = 500 + 10;
struct edge{
int to,cap,rev;
edge(){}
edge(int _to,int _cap,int _rev):to(_to),cap(_cap),rev(_rev){}
};
vector<edge>G[MAX_V];
int level[MAX_V];
int iter[MAX_V];
void add(int from,int to,int cap)
{
G[from].push_back(edge(to,cap,G[to].size()));
G[to].push_back(edge(from,0,G[from].size()-1));
}
void bfs(int s)
{
memset(level,-1,sizeof(level));
queue<int>que;
level[s] = 0;
que.push(s);
while(!que.empty()){
int f = que.front();
que.pop();
for( int i = 0; i < G[f].size(); i++ ){
edge &e = G[f][i];
if(e.cap > 0 && level[e.to] == -1){
level[e.to] = level[f] + 1;
que.push(e.to);
}
}
}
}
int dfs(int v,int t,int f)
{
if(v == t)return f;
for( int &i = iter[v]; i < G[v].size(); i++ ){
edge &e = G[v][i];
if(e.cap > 0 && level[v] < level[e.to]){
int d = dfs(e.to,t,min(e.cap,f));
if(d > 0){
e.cap -= d;
G[e.to][e.rev].cap += d;
return d;
}
}
}
return 0;
}
int max_flow(int s,int t)
{
int flow = 0;
for(;;){
bfs(s);
if(level[t] == -1)return flow;
memset(iter,0,sizeof(iter));
int f;
while((f = dfs(s,t,INF)) > 0)
flow += f;
}
}
int nxt[MAX_V];
int main()
{
int n,m;
scanf("%d%d",&n,&m);
int s = 0, t = n + n + 1;
for( int i = 1; i <= n; i++ )add(s,i,1),add(i+n,t,1);
for( int i = 1; i <= m; i++ ){
int u,v;
scanf("%d%d",&u,&v);
add(u,v+n,1);
}
vector<int>a;
int ret = max_flow(s,t);

for( int i = 1; i <= n; i++ ){
for( int j = 0; j < G[i].size(); j++ ){
if(G[i][j].to > n && G[i][j].cap == 0){
nxt[i] = G[i][j].to - n;
break;
}
}
}
//找出路径起点
for( int i = 1; i <= n; i++ ){
for( int j = 0; j < G[i+n].size(); j++ ){
if(G[i+n][j].to == t && G[i+n][j].cap == 1){
a.pb(i);
break;
}
}
}
for( int i = 0; i < a.size(); i++ ){
printf("%d ",a[i]);
while(nxt[a[i]]){
printf("%d ",nxt[a[i]]);
a[i] = nxt[a[i]];
}
puts("");
}
printf("%d\n",n - ret);
return 0;
}