poj2240Arbitrage 判断是否有负环

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/*

题目：

       问周游一圈后能不能换钱得到利润

分析：

       有向图是否处在负环问题，因为涉及每个顶点都可能存在负环，

       直接用flord算法做即可

*/

#include <iostream>

#include <string>

#include <cstring>

using namespace std;

#define X 35

string city[X],s1,s2;

double map[X][X];

int main()

{

       freopen("sum.in","r",stdin);

       freopen("sum.out","w",stdout);

       int m,n,x,y,cnt = 0;

       double dis;

       while(cin>>n,n)

       {

              memset(map,0,sizeof(map));

              for(int i=0;i<n;i++)

              {

                     cin>>city[i];

                     map[i][i] = 1;

              }

              cin>>m;

              for(int i=0;i<m;i++)

              {

                     cin>>s1>>dis>>s2;

                     for(int j=0;j<n;j++)//找出起点终点

                     {

                            if(s1==city[j])

                                   x = j;

                            if(s2==city[j])

                                   y = j;

                     }

                     map[x][y] = dis;     //有向图的路径长度

              }

              for(int k=0;k<n;k++)     //fload算法

                     for(int i=0;i<n;i++)

                            for(int j=0;j<n;j++)

                                   if(map[i][j]<map[i][k]*map[k][j])              //松弛操作

                                          map[i][j] = map[i][k]*map[k][j];

              bool flag = true;

              for(int i=0;i<n;i++)

                     if(map[i][i]>1)       //如果存在负环

                            flag = false;

              if(!flag)

                     printf("Case %d: Yes\n",++cnt);

              else

                     printf("Case %d: No\n",++cnt);

       }

       return 0;

}