Currency Exchange---poj1860 ( spfa, 回路，最长路）

#include <iostream>
#include <cstdio>
#include <cstring>
#include <stdlib.h>
#include <math.h>
#include <queue>
#include <algorithm>
using namespace std;
#define N 210
#define INF 0xfffffff
double dist[N], V;
int cnt, Head[N], num[N], vis[N];
int n, m, s;

struct Edge
{
    int v, next;
    double r, c;
}e[N];

void Add(int u, int v, double r, double c)
{
    e[cnt].v = v;
    e[cnt].r = r;
    e[cnt].c = c;
    e[cnt].next = Head[u];
    Head[u] = cnt++;
}

bool spfa()///spfa模板；
{
    memset(vis, 0, sizeof(vis));
    memset(num, 0, sizeof(num));
    queue<int>Q;
    vis[s] = 1;
    dist[s] = V;
    Q.push(s);
    num[s]++;
    while(Q.size())
    {
        int p=Q.front();
        Q.pop();
        vis[p] = 0;
        for(int i=Head[p]; i!=-1; i=e[i].next)
        {
            int q = e[i].v;
            if(dist[q] < (dist[p] - e[i].c) * e[i].r)///注意松弛的变化；
            {
                dist[q] = (dist[p] - e[i].c) * e[i].r;
                if(!vis[q])
                {
                    vis[q] = 1;
                    Q.push(q);
                    num[q] ++;
                    if(num[q]>n)
                        return true;///存在正权回路；
                }
            }
        }

    }
    if(dist[s]>V)///最长路后，实现了增值；
        return true;
    return false;
}

int main()
{
    int a, b;
    double rab, rba, cab, cba;
    while(scanf("%d%d%d%lf", &n, &m, &s, &V)!=EOF)
    {
        cnt = 0;
        memset(Head, -1, sizeof(Head));
        memset(dist, 0, sizeof(dist));
        for(int i=1; i<=m; i++)
        {
            scanf("%d%d%lf%lf%lf%lf", &a, &b, &rab, &cab, &rba, &cba);
            Add(a, b, rab, cab);
            Add(b, a, rba, cba);
        }
        if( spfa() )
            printf("YES\n");
        else
            printf("NO\n");
    }
    return 0;
}