Bellman-ford算法的反向应用－－正循环检查

/** \brief poj 1860 Bellman-Ford
 *
 * \param date 2014/7/24
 * \param state AC
 * \return memory 708K time 141ms
 *
 */

#include <iostream>
#include <fstream>
#include <cstring>

using namespace std;

struct RateAndCom
{
//public:
    int a;
    int b;
    double rate;
    double Com;
};//Map[MAXN];

const int MAXN=101;
RateAndCom Map[101*2];
double dis[MAXN];

int N;//货币种数
int M;//兑换点数量
int S;//持有第s种货币
double V;//第s种货币本金
int allEdge;

bool Bellman_Ford()
{
    memset(dis,0,sizeof(dis));
    dis[S]=V;
    /*relax*/
    bool flag;
    for(int i=1;i<=N-1;i++)
    {
        flag=false;
        for(int j=0;j<allEdge;j++)
            if(dis[Map[j].b] < (dis[Map[j].a]-Map[j].Com)*Map[j].rate)
        {
            dis[Map[j].b] = (dis[Map[j].a]-Map[j].Com)*Map[j].rate;
            flag=true;
        }
        if(!flag)
            break;
    }

    for(int k=0;k<allEdge;k++)
        if(dis[Map[k].b] < (dis[Map[k].a]-Map[k].Com)*Map[k].rate)
        return true;

    return false;
}

int main()
{
    //cout << "Hello world!" << endl;
    //freopen("input.txt","r",stdin);
    //while(scanf("%d %d %d %f",&N,&M,&S,&V)!=EOF)
    while(cin>>N>>M>>S>>V)
    {
        allEdge=0;
        for(int i=0;i<M;i++)
        {
            int a,b;
            double Rab;
            double Cab;
            double Rba;
            double Cba;
            //cin>>a>>b>>Map[a][b].rate>>Map[a][b].Commission
            //>>Map[b][a].rate>>Map[b][a].Commission;
            cin>>a>>b>>Rab>>Cab>>Rba>>Cba;
            Map[allEdge].a=a;
            Map[allEdge].b=b;
            Map[allEdge].rate=Rab;
            Map[allEdge].Com=Cab;
            allEdge++;
            Map[allEdge].a=b;
            Map[allEdge].b=a;
            Map[allEdge].rate=Rba;
            Map[allEdge].Com=Cba;
            allEdge++;
        }
        //Bellman-Ford
        if(Bellman_Ford())
            cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}