luogu1345 [USACO5.4]奶牛的电信Telecowmunication

对于每个点，把它拆成有权值为1的边相连的两个点，原边是inf。

边的起点统一加n，ss也加n

这就成了最小割

#include <iostream>
#include <cstring>
#include <cstdio>
#include <queue>
using namespace std;
int n, m, ss, tt, hea[205], cnt, uu, vv, maxFlow=0, lev[205];
const int oo=0x3f3f3f3f;
queue<int> d;
struct Edge{
	int too, nxt, val;
}edge[4005];
void add_edge(int fro, int too, int val){
	edge[cnt].nxt = hea[fro];
	edge[cnt].too = too;
	edge[cnt].val = val;
	hea[fro] = cnt++;
}
void addEdge(int fro, int too, int val){
	add_edge(fro, too, val);
	add_edge(too, fro, 0);
}
bool bfs(){
	memset(lev, 0, sizeof(lev));
	d.push(ss);
	lev[ss] = 1;
	while(!d.empty()){
		int x=d.front();
		d.pop();
		for(int i=hea[x]; i!=-1; i=edge[i].nxt){
			int t=edge[i].too;
			if(!lev[t] && edge[i].val>0){
				lev[t] = lev[x] + 1;
				d.push(t);
			}
		}
	}
	return lev[tt]!=0;
}
int dfs(int x, int lim){
	if(x==tt)	return lim;
	int addFlow=0;
	for(int i=hea[x]; i!=-1 && addFlow<lim; i=edge[i].nxt){
		int t=edge[i].too;
		if(lev[t]==lev[x]+1 && edge[i].val>0){
			int tmp=dfs(t, min(lim-addFlow, edge[i].val));
			edge[i].val -= tmp;
			edge[i^1].val += tmp;
			addFlow += tmp;
		}
	}
	return addFlow;
}
void dinic(){
	while(bfs())	maxFlow += dfs(ss, oo);
}
int main(){
	memset(hea, -1, sizeof(hea));
	cin>>n>>m>>ss>>tt;
	ss += n;
	for(int i=1; i<=n; i++)
		addEdge(i, i+n, 1);
	for(int i=1; i<=m; i++){
		scanf("%d %d", &uu, &vv);
		addEdge(uu+n, vv, oo);
		addEdge(vv+n, uu, oo);
	}
	dinic();
	cout<<maxFlow<<endl;
	return 0;
}