poj1860 Currency Exchange

思路：

Bellman-Ford判断是否含有经过源点的正权回路。

实现：

#include <iostream>
#include <cstdio>
using namespace std;

const int MAXN = 105, MAXM = 105;

int n, m, s;
double d, dist[MAXN];
struct edge
{
    int from, to;
    double c, r;
};
edge es[2 * MAXM];

bool check(int s, double d)
{
    for (int i = 1; i <= n; i++) dist[i] = 0;
    dist[s] = d;
    bool update = true;
    while (update)
    {
        if (dist[s] > d) return true;
        update = false;
        for (int i = 1; i <= 2 * m; i++)
        {
            edge & e = es[i];
            if (dist[e.from] > 0 && dist[e.to] < (dist[e.from] - e.c) * e.r)
            {
                dist[e.to] = (dist[e.from] - e.c) * e.r;
                update = true;
            }
        }
    }
    return dist[s] > d;
}

int main()
{
    cin >> n >> m >> s >> d;
    for (int i = 1; i < 2 * m; i += 2)
    {
        cin >> es[i].from >> es[i].to >> es[i].r >> es[i].c;
        cin >> es[i + 1].r >> es[i + 1].c;
        es[i + 1].from = es[i].to;
        es[i + 1].to = es[i].from;
    }
    if (!check(s, d)) puts("NO");
    else puts("YES");
}