poj 1860Currency Exchange

思路：

bellman-ford检验正回路

代码：

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
struct infor{
	int from;
	int to;
	double r;
	double c;
};
infor es[20008];
double dis[200],v;
int enu,n,s;
bool bellman()
{
	memset(dis,0,sizeof(dis));
	dis[s]=v;
	for(int i=0;i<n;i++){
		for(int j=0;j<enu;j++){
			infor e=es[j];
			if(dis[e.to]<(dis[e.from]-e.c)*e.r){
				dis[e.to]=(dis[e.from]-e.c)*e.r;
				if(i==n-1)return true;
			}
		}
	}
	return false;
}
int main()
{
	int m,a,b;
	enu=0;
	double rab,cab,rba,cba;
	cin>>n>>m>>s>>v;
	for(int i=0;i<m;i++){
		cin>>a>>b>>rab>>cab>>rba>>cba;
		es[enu].from=a;
		es[enu].to=b;
		es[enu].r=rab;
		es[enu++].c=cab;
		es[enu].from=b;
		es[enu].to=a;
		es[enu].r=rba;
		es[enu++].c=cba;
	}
	if(bellman())
		cout<<"YES"<<endl;
	else
		cout<<"NO"<<endl;
}