# POJ 1860 Bellman-Ford反向 求正权回路

``````//题目的意思就是找正权回路
//我们知道Bellman-Ford可以标记负权回路
//我们不妨把Belliman-Ford的不等式反写
//就可以标记正权回路了
#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int maxe=210;
const int maxv=110;
int cnt;
double dis[maxv];
struct Edge {
int from,to;
double rate,commission;
} edge[maxe];
void init() {
cnt=0;
memset(dis,0,sizeof dis);
}
void addedge(int from,int to,double rate,double commission) {
edge[cnt].from=from;
edge[cnt].to=to;
edge[cnt].rate=rate;
edge[cnt++].commission=commission;
}
bool bellman_ford(int s,double v,int n,int cnt,Edge edge[],double dist[]) {
bool flag;
dist[s]=v;
for(int j=0; j<n+1; j++) {
flag=false;
for(int i=0; i<cnt; i++) {
if(dist[edge[i].to]<(dist[edge[i].from]-edge[i].commission)*edge[i].rate) {
dist[edge[i].to]=(dist[edge[i].from]-edge[i].commission)*edge[i].rate;
flag=true;
}
}
if(flag==false)
break;
}
return flag;
}

int main(void) {
#ifndef ONLINE_JUDGE
freopen("E:\\input.txt","r",stdin);
#endif // ONLINE_JUDGE
ios::sync_with_stdio(false);
int n,m,s,a,b;
double v,rab,cab,rba,cba;
while(cin>>n>>m>>s>>v) {
init();
for(int i=0; i<m; i++) {
cin>>a>>b>>rab>>cab>>rba>>cba;

}
if(bellman_ford(s,v,n,cnt,edge,dis))
cout<<"YES"<<endl;
else
cout<<"NO"<<endl;
}
return 0;
}
``````

原文作者：Bellman - ford算法