POJ 2240 Arbitrage（判正环）

http://poj.org/problem?id=2240

题意：
货币兑换，判断最否是否能获利。

思路：
又是货币兑换题，Belloman-ford和floyd算法都可以的。

 #include<iostream>
 #include<algorithm>
 #include<string>
 #include<cstring>
 #include<map>
 using namespace std;

 const int maxn =  + ;

 int n, m;
 string s1,s2;

 map<string, int> p;

 struct
 {
     int u, v;
     double rate;
 }edge[maxn];

 double d[];

 bool bellman(int s)
 {
     memset(d, , sizeof(d));
     d[s] = ;
     for (int i = ; i < n; i++)
     {
         bool flag = false;
         for (int j = ; j < m; j++)
         {
             if (d[edge[j].v] < d[edge[j].u] * edge[j].rate)
             {
                 d[edge[j].v] = d[edge[j].u] * edge[j].rate;
                 flag = true;
             }
         }
         if (!flag)  break;
     }
     for (int j = ; j < m; j++)
         if(d[edge[j].v] < d[edge[j].u] * edge[j].rate)
         return true;
     return false;
 }

 int main()
 {
     ios::sync_with_stdio(false);
     //freopen("D:\\txt.txt", "r", stdin);
     double r;
     int kase = ;
     while (~scanf("%d", &n) && n)
     {
         for (int i = ; i < n; i++)
         {
             cin >> s1;
             p[s1] = i;
         }
         scanf("%d", &m);
         for (int i = ; i < m; i++)
         {
             cin >> s1 >> r >> s2;
             edge[i].u = p[s1];
             edge[i].v = p[s2];
             edge[i].rate = r;
         }
         printf("Case %d: ", ++kase);
         bool flag = false;
         for (int i = ; i < n; i++)
         {
             if (bellman(i))
             {
                 printf("Yes\n");
                 flag = true;
                 break;
             }
         }
         if (!flag)
             printf("No\n");
     }
     return ;
 }

Belloman-ford

 #include<iostream>
 #include<algorithm>
 #include<string>
 #include<cstring>
 #include<map>
 using namespace std;

 const int maxn =  + ;
 const int INF = ;

 int n, m;
 string s1,s2;

 map<string, int> p;

 double d[maxn][maxn];

 void floyd()
 {
     for (int k = ; k < n; k++)
     for (int i = ; i < n; i++)
     for (int j = ; j < n; j++)
         d[i][j] = max(d[i][j], d[i][k] * d[k][j]);
 }

 int main()
 {
     ios::sync_with_stdio(false);
     //freopen("D:\\txt.txt", "r", stdin);
     double r;
     int kase = ;
     while (~scanf("%d", &n) && n)
     {
         memset(d, INF, sizeof(d));
         for (int i = ; i < n; i++)
         {
             cin >> s1;
             p[s1] = i;
             d[i][i] = ;   //一开始的汇率为1
         }
         scanf("%d", &m);
         for (int i = ; i < m; i++)
         {
             cin >> s1 >> r >> s2;
             d[p[s1]][p[s2]] = r;
         }
         floyd();
         printf("Case %d: ", ++kase);
         bool flag = false;
         for (int i = ; i < n; i++)
         {
             if (d[i][i]>)   //如果自身汇率大于1，说明可以套利
             {
                 printf("Yes\n");
                 flag = true;
                 break;
             }
         }
         if (!flag)
             printf("No\n");
     }
     return ;
 }

floyd