最大流【模板】

洛谷P3376

https://www.luogu.com.cn/problem/P3376

Dinic算法

#include<bits/stdc++.h>
using namespace std;
#define ll long long
const int maxn=5e2+7;
const int maxm=2e5+7;
const ll INF=1e18;
int n,m,s,t;
//struct node{
//	int e;
//	ll v;
//	int nxt;
//}edge[maxm];
int e[maxm],nxt[maxm];
ll val[maxm];
int dis[maxn];
int head[maxn];
int cnt=0;
int cur[maxn];
ll minn(ll a, ll b){
	return a>b?b:a;
}
void add(int u,int v,int w){
//	edge[cnt].e=v;
//	edge[cnt].v=w;
//	edge[cnt].nxt=head[u];
	e[cnt]=v;
	val[cnt]=w;
	nxt[cnt]=head[u];
	head[u]=cnt++;
}
inline bool bfs(){
for(int i=1;i<=n;i++) dis[i]=-1;
	dis[s]=0;
	queue<int> q;
	q.push(s);
	while(!q.empty()){
		int r=q.front();
		q.pop();
//		for(int i=head[r];i!=0;i=edge[i].nxt){
		for(int i=head[r];i!=-1;i=nxt[i]){
//			int j=edge[i].e;
			int j=e[i];
//			if(dis[j]==-1&&edge[i].v){
			if(dis[j]==-1&&val[i]>0){
				dis[j]=dis[r]+1;
				q.push(j);
				if(j==t) return 1;
			}
		}
	}
	return 0;
}
ll dfs(int u,ll flo){
	if(u==t) return flo;
	ll ans=0;
//	for(int i=cur[u];i!=0;i=edge[i].nxt){
	for(ll i=cur[u];i!=-1&&flo;i=nxt[i]){
//		if(detla<=0) break;
		cur[u]=i;
//		int v=edge[i].e;
		int v=e[i];
//		if(dis[v]==(dis[u]+1)&&edge[i].v){
		if(dis[v]==(dis[u]+1)&&val[i]){
//			ll b=dfs(v,minn(edge[i].v,detla));
			ll b=dfs(v,minn(val[i],flo));
			if(!b) dis[b]=-1;//剪枝
//			edge[i].v-=b;edge[i^1].v+=b;
			val[i]-=b; val[i^1]+=b;
			flo-=b;
			ans+=b;
		}	
	}
	return ans;	
}
ll dini(){
	ll ans=0;
	while(bfs()){
		for(int i=1;i<=n;i++) cur[i]=head[i];
		ans+=dfs(s,INF);
	}
	return ans;		
}
int main(){
	memset(head,-1,sizeof head);
	scanf("%d%d%d%d",&n,&m,&s,&t);
	for(int i=0;i<m;i++){
		int a,b;
		int  c;
		scanf("%d%d%d",&a,&b,&c);
		add(a,b,c);
		add(b,a,0);
	}	
	printf("%lld",dini());	
	return 0;
}