POJ-2240 Arbitrage

来源：互联网发布：网络语666是什么意思编辑：程序博客网时间：2024/06/08 05:32

题目链接：https://vjudge.net/problem/POJ-2240

又是一道判断负环的题，这次用spfa做，先用了一般的bfs版的spfa

#include<iostream>#include<cstdio>#include<cstring>#include<algorithm>#include<cmath>#include<queue>#include<map>using namespace std;const int N=30+2;int n,m;double g[N][N];double d[N];bool vis[N];bool spfa(){int cnt[N];for(int i=0;i<n;i++)d[i]=0.0;memset(vis,false,sizeof(vis));memset(cnt,0,sizeof(cnt));d[0]=100;vis[0]=true;cnt[0]=1;queue<int> q;q.push(0);while(!q.empty()){int u=q.front();vis[u]=false;q.pop();for(int i=0;i<n;i++)if(d[i]<d[u]*g[u][i]){d[i]=d[u]*g[u][i];if(!vis[i]){cnt[i]++;if(cnt[i]>=n) return true;q.push(i);}vis[i]=true;}}return false;}int main(){int kase=0;while(~scanf("%d",&n)&&n){map<string,int> mm;string a,b;double c;for(int i=0;i<n;i++){cin>>a;mm[a]=i;}for(int i=0;i<n;i++)for(int j=0;j<n;j++)g[i][j]=0.0;scanf("%d",&m);for(int i=0;i<m;i++){cin>>a>>c>>b;g[mm[a]][mm[b]]=c;}printf("Case %d: ",++kase);if(spfa()) printf("Yes\n");else printf("No\n");}return 0;}

然后听说dfs优化的spfa判断负环更好些，结果是对这道题来说时间差不多

#include<iostream>#include<cstdio>#include<cstring>#include<algorithm>#include<cmath>#include<queue>#include<map>using namespace std;const int N=30+2;int n,m;double g[N][N];double d[N];bool vis[N],flag;void spfa(int u){if(flag) return;for(int i=0;i<n;i++)if(d[i]<d[u]*g[u][i]){if(vis[i]){flag=true;return;}else{d[i]=d[u]*g[u][i];vis[i]=true;spfa(i);vis[i]=false;}}}int main(){int kase=0;while(~scanf("%d",&n)&&n){map<string,int> mm;string a,b;double c;for(int i=0;i<n;i++){cin>>a;mm[a]=i;}for(int i=0;i<n;i++)for(int j=0;j<n;j++)g[i][j]=0.0;scanf("%d",&m);for(int i=0;i<m;i++){cin>>a>>c>>b;g[mm[a]][mm[b]]=c;}for(int i=0;i<n;i++)d[i]=0.0;d[0]=100;memset(vis,false,sizeof(vis));vis[0]=true;flag=false;spfa(0);printf("Case %d: ",++kase);if(flag) printf("Yes\n");else printf("No\n");}return 0;}

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