Currency Exchange POJ - 1860 spfa判断正环

Posted 2021-03-13 qingyuyyyyy

//spfa 判断正环 
#include<iostream>
#include<queue>
#include<cstring>
using namespace std;
const int N=1e4;
const int INF=2e9;
int h[N],to[N],ne[N],idx;
double r[N],c[N];
int n, m,X;
double V;
void add(int u,int v,double r1,double c1)
{
    to[idx]=v;
    r[idx]=r1;
    c[idx]=c1;
    ne[idx]=h[u];
    h[u]=idx++;
}
double dist[N];
int times[N];
bool vis[N];
bool spfa(int st)
{
    memset(vis,0,sizeof vis);
    memset(times,0,sizeof times);
    for(int i=1;i<=n;i++)
        dist[i]=-INF;
    queue<int>q;
    q.push(st);
    vis[st]=1;
    dist[st]=V;
    while(q.size())
    {
        int u=q.front();
        q.pop();
        vis[u]=false;
        for(int i=h[u];~i;i=ne[i])
        {
            int v=to[i];
            if(dist[v]<(dist[u]-c[i])*r[i])
            {
                dist[v]=(dist[u]-c[i])*r[i];
                if(!vis[v])
                {
                    q.push(v);
                    vis[v]=1;
                    if(++times[v]>n)
                        return true;
                }
            }
        }
    }
    return false;
}
int main()
{
    while(cin>>n>>m>>X>>V)
    {
        idx=0;
        memset(h,-1,sizeof h);
        for(int i=1;i<=m;i++)
        {
            int u,v;
            double r1,c1,r2,c2;
            cin>>u>>v>>r1>>c1>>r2>>c2;
            add(u,v,r1,c1);
            add(v,u,r2,c2);
        }
        if(spfa(X))
            cout<<"YES"<<endl;
        else
            cout<<"NO"<<endl;
    }
}