POJ1860-Currency Exchange-判正环

两种货币的交换可以当成两条边，建图后跑Bellman_ford算法就好了。

Bellman_ford算法可以用来处理负边权，所以可以判断是否存在负环。反过来就可以判断是否存在正环。

/*--------------------------------------------------------------------------------------*/
//        Helica's header 
//        Second Edition
//        2015.11.7
//
#include <algorithm>
#include <iostream>
#include <cstring>
#include <ctype.h>
#include <cstdlib>
#include <cstdio>
#include <vector>
#include <string>
#include <queue>
#include <stack>
#include <cmath>
#include <set>
#include <map>

//debug function for a N*M array 
#define debug_map(N,M,G) printf("
");for(int i=0;i<(N);i++)
{for(int j=0;j<(M);j++){
printf("%d",G[i][j]);}printf("
");}                
//debug function for int,float,double,etc.
#define debug_var(X) cout<<#X"="<<X<<endl;
/*--------------------------------------------------------------------------------------*/
using namespace std;

const int maxn = 100+10;
int N,M,T,S;
int tol;
double V;

struct Edge
{
    int u,v;
    double c,r;
}E[10*maxn];

double dist[maxn];

bool bellman_ford(int start,int n)
{
    memset(dist,0,sizeof dist);
    dist[start] = V;

    for(int i=1;i<n;i++)
    {
        bool flag = false;
        for(int j=0;j<tol;j++)if(dist[E[j].v] < (dist[E[j].u]-E[j].c)*E[j].r )
        {
            dist[E[j].v] = (dist[E[j].u]-E[j].c)*E[j].r;
           flag = true;    
        }
        if(!flag) return false;
    }

    for(int j=0;j<tol;j++)
    {
        if(dist[E[j].v] < (dist[E[j].u]-E[j].c)*E[j].r)
            return true;
    }
    return false;
}

int main()
{
    while(~scanf("%d%d%d%lf",&N,&M,&S,&V))
    {
        int a,b;
        double r1,r2,c1,c2;
        tol = 0;
        for(int i=0;i<M;i++)
        {
            scanf("%d%d%lf%lf%lf%lf",&a,&b,&r1,&c1,&r2,&c2);
            E[tol].u = a;E[tol].v = b;
            E[tol].c = c1;E[tol++].r = r1;
            E[tol].u = b;E[tol].v = a;
            E[tol].c = c2;E[tol++].r = r2;
        }
        if(bellman_ford(S,N))
            printf("YES
");
        else
            printf("NO
");

    }    
}