题目链接：http://poj.org/problem?id=1860

水一水，

#include <stdio.h> 
#include <vector> 
#include <algorithm>
using namespace std;

//POJ 1860 Currency Exchange

struct Edge
{
    int in, out;
    double rate, commision;
    Edge() {}
    Edge(double r, double c) :rate(r), commision(c) {}
};

int main()
{
    int N, M, S;
    double V;
    while (~scanf("%d%d%d%lf", &N, &M, &S, &V))
    {
        vector<Edge> edges(2 * M);
        vector<double> currency(N);
        S--;
        currency[S] = V;
        for (int i = 0; i < 2 * M;)
        {
            int A, B;
            double R_AB, C_AB, R_BA, C_BA;
            scanf("%d%d%lf%lf%lf%lf", &A, &B, &R_AB, &C_AB, &R_BA, &C_BA);
            A--; B--;
            edges[i].in = A;
            edges[i].out = B;
            edges[i].rate = R_AB;
            edges[i].commision = C_AB;
            i++;
            edges[i].in = B;
            edges[i].out = A;
            edges[i].rate = R_BA;
            edges[i].commision = C_BA;
            i++;
        }
        bool flag = false;
        for (int k = 1; k < N; ++k)
            for (int i = 0; i < 2 * M; ++i)
            {
                const Edge& edge = edges[i];
                int in = edge.in, out = edge.out;
                double rate = edge.rate, commision = edge.commision;
                double exchange = (currency[in] - commision)*rate;
                if (exchange > currency[out])
                    currency[out] = exchange;
            }
        for (int i = 0; i < 2 * M; ++i)
        {
            const Edge& edge = edges[i];
            int in = edge.in, out = edge.out;
            double rate = edge.rate, commision = edge.commision;
            double exchange = (currency[in] - commision)*rate;
            if (exchange > currency[out])
            {
                flag = true;
                break;
            }
        }
        if (flag)
            printf("YES\n");
        else printf("NO\n");

    }
    system("pause");
    return 0;
}