Currency Exchange

主要找正环：

 #include<cstdio>
  #include<cstring>
  #include<cstdlib>
  #include<algorithm>
  #include<iostream>
  #include<queue>
  #define MAXN 52000
  using namespace std;
  const int INF=-<<;
  bool vis[MAXN];
  int head[MAXN];
  int next[MAXN];
  int s;
  double dis[MAXN];
  struct node
  {
      int u,v;
      double r,c;
  }p[MAXN];
  int e,n,m;
  int cnt[MAXN];
  double v;
  void addnode(int u,int v,double r,double c)
  {
      p[e].u=u;
      p[e].v=v;
      p[e].r=r;
      p[e].c=c;
      next[e]=head[u];
      head[u]=e++;
  }
  bool relax(int u,int v,double r,double c)
  {
      if(dis[v]<(dis[u]-c)*r)
      {
          dis[v]=(dis[u]-c)*r;
          return true;
      }
      return false;
  }
  bool spfa(int rc)
  {
      memset(vis,false,sizeof(vis));
      memset(cnt,,sizeof(cnt));
      for(int i=;i<=n;i++)
          dis[i]=;
      dis[rc]=v;
      vis[rc]=true;
      queue<int>q;
      q.push(rc);
      ++cnt[rc];
      while(!q.empty()){
          int pre=q.front();
          q.pop();
          vis[pre]=false;
          for(int i=head[pre];i+;i=next[i])
          {
              if(relax(pre,p[i].v,p[i].r,p[i].c)&&!vis[p[i].v]){
                  if((++cnt[p[i].v])>n) return false;
                  q.push(p[i].v);
                  vis[p[i].v]=true;
              }
          }
      }
      return true;
  }
  int main()
  {
      memset(head, -, sizeof(head));
      memset(next, -, sizeof(next));
      e=;
      int a,b;
      double r1,c1,r2,c2;
      scanf("%d%d%d%lf",&n,&m,&s,&v);
      for(int i=;i<m;i++){
      scanf("%d%d%lf%lf%lf%lf",&a,&b,&r1,&c1,&r2,&c2);
      addnode(a,b,r1,c1);
      addnode(b,a,r2,c2);
      }
      if(!spfa(s)) printf("YES\n");
      else printf("NO\n");
      return ;
  }