poj2240 Arbitrage

思路：

有向图判负环。

实现：

（1）spfa

 #include <iostream>
 #include <map>
 #include <string>
 #include <cmath>
 #include <queue>
 using namespace std;
 const int INF = 0x3f3f3f3f;
 const int MAXN = ;
 double G[MAXN][MAXN], d[MAXN];
 bool in[MAXN];
 int n, m, num[MAXN];
 bool spfa(int s)
 {
     queue<int> q;
     d[s] = ;
     q.push(s);
     in[s] = true;
     while (!q.empty())
     {
         int tmp = q.front(); q.pop();
         in[tmp] = false;
         for (int i = ; i <= n; i++)
         {
             if (d[tmp] + G[tmp][i] < d[i])
             {
                 d[i] = d[tmp] + G[tmp][i];
                 if (!in[i]) { in[i] = true; q.push(i); }
                 num[i]++;
                 if (num[i] > n) return true;
             }
         }
     }
     return false;
 }
 int main()
 {
     map<string, int> mp;
     string s, t;
     double x;
     int Kase = ;
     while (cin >> n, n)
     {
         for (int i = ; i <= n; i++)
         {
             for (int j = ; j <= n; j++)
             {
                 G[i][j] = INF;
             }
         }
         fill(d, d + n + , INF);
         fill(in, in + n + , );
         fill(num, num + n + , );
         mp.clear();
         for (int i = ; i <= n; i++)
         {
             cin >> s;
             mp[s] = i;
         }
         cin >> m;
         for (int i = ; i <= m; i++)
         {
             cin >> s >> x >> t;
             G[mp[s]][mp[t]] = -log(x);
         }
         for (int i = ; i <= n; i++) G[][i] = ;
         cout << "Case " << Kase++ << ": ";
         if (spfa()) cout << "Yes" << endl;
         else cout << "No" << endl;
     }
     return ;
 }

（2）floyd

 #include <iostream>
 #include <map>
 #include <string>
 #include <cmath>
 using namespace std;
 const int INF = 0x3f3f3f3f;
 const int MAXN = ;
 double G[MAXN][MAXN];
 int n, m;
 
 int main()
 {
     map<string, int> mp;
     string s, t;
     double x;
     int Kase = ;
     while (cin >> n, n)
     {
         for (int i = ; i <= n; i++)
         {
             for (int j = ; j <= n; j++)
             {
                 if (i == j) G[i][j] = ;
                 else G[i][j] = INF;
             }
         }
         mp.clear();
         for (int i = ; i <= n; i++)
         {
             cin >> s;
             mp[s] = i;
         }
         cin >> m;
         for (int i = ; i <= m; i++)
         {
             cin >> s >> x >> t;
             G[mp[s]][mp[t]] = -log(x);
         }
         for (int k = ; k <= n; k++)
         {
             for (int i = ; i <= n; i++)
             {
                 for (int j = ; j <= n; j++)
                 {
                     if (G[i][k] + G[k][j] < G[i][j])
                         G[i][j] = G[i][k] + G[k][j];
                 }
             }
         }
         bool flg = false;
         for (int i = ; i <= n; i++)
         {
             if (G[i][i] < ) { flg = true; break; }
         }
         cout << "Case " << Kase++ << ": ";
         if (flg) cout << "Yes" << endl;
         else cout << "No" << endl;
     }
     return ;
 }