poj 3259Wormholes

思路：

blleman—ford检验负回路

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;
}