[洛谷P3376]模板网络最大流

Posted 2020-09-30 Mrsrz

题目大意：略。

解题思路：最大流木板题，以下是Dinic算法的代码。

C++ Code：

 

#include<stdio.h>
#include<cctype>
#include<vector>
#include<algorithm>
#include<cstring>
#include<queue>
using namespace std;
#define INF 0x3f3f3f3f
char buf[40000020];
int bufpos,n,m,s,t,level[10002],iter[10002];
struct edges{
	int to,cap,rev;
	edges(int t,int c,int r):to(t),cap(c),rev(r){}
};
vector<edges>G[10002];
queue<int>q;
inline void init(){
    buf[fread(buf,1,40000000,stdin)]=‘\0‘;
    bufpos=0;
}
inline int readint(){
	int p=0;
    for(;!isdigit(buf[bufpos]);bufpos++);
    for(;isdigit(buf[bufpos]);bufpos++)
    p=p*10+buf[bufpos]-‘0‘;
    return p;
}
inline void addedge(int from,int to,int cap){
	G[from].push_back(edges(to,cap,G[to].size()));
	G[to].push_back(edges(from,0,G[from].size()-1));
}
void bfs(int s){
	memset(level,-1,sizeof(level));
	level[s]=0;
	q.push(s);
	while(!q.empty()){
		int u=q.front();
		q.pop();
		for(int i=0;i<G[u].size();++i){
			edges& e=G[u][i];
			if(level[e.to]<0&&e.cap>0){
				level[e.to]=level[u]+1;
				q.push(e.to);
			}
		}
	}
}
int dfs(int u,int t,int f){
	if(u==t)return f;
	for(int& i=iter[u];i<G[u].size();++i){
		edges& e=G[u][i];
		if(e.cap>0&&level[e.to]>level[u]){
			int d=dfs(e.to,t,min(f,e.cap));
			if(d){
				e.cap-=d;
				G[e.to][e.rev].cap+=d;
				return d;
			}
		}
	}
	return 0;
}
int max_flow(int s,int t){
	int flow=0;
	while(1){
		bfs(s);
		if(level[t]<0)return flow;
		memset(iter,0,sizeof(iter));
		int f;
		while(f=dfs(s,t,INF))flow+=f;
	}
}
int main(){
	#ifdef DEBUG
	freopen("input.txt","r",stdin);
	freopen("output.txt","w",stdout);
	#endif
	init();
	n=readint(),m=readint(),s=readint(),t=readint();
	for(int i=1;i<=m;++i){
		int u=readint(),v=readint(),cap=readint();
		addedge(u,v,cap);
	}
	printf("%d\n",max_flow(s,t));
	return 0;
}