poj 2240 Arbitrage(Bellman-Ford||Floyd)(中等)

Arbitrage

Time Limit: 1000MS Memory Limit: 65536KTotal Submissions: 17647 Accepted: 7443

Description

Arbitrage is the use of discrepancies in currency exchange rates to transform one unit of a currency into more than one unit of the same currency. For example, suppose that 1 US Dollar buys 0.5 British pound, 1 British pound buys 10.0 French francs, and 1 French franc buys 0.21 US dollar. Then, by converting currencies, a clever trader can start with 1 US dollar and buy 0.5 * 10.0 * 0.21 = 1.05 US dollars, making a profit of 5 percent. 

Your job is to write a program that takes a list of currency exchange rates as input and then determines whether arbitrage is possible or not. 

Input

The input will contain one or more test cases. Om the first line of each test case there is an integer n (1<=n<=30), representing the number of different currencies. The next n lines each contain the name of one currency. Within a name no spaces will appear. The next line contains one integer m, representing the length of the table to follow. The last m lines each contain the name ci of a source currency, a real number rij which represents the exchange rate from ci to cj and a name cj of the destination currency. Exchanges which do not appear in the table are impossible. 
Test cases are separated from each other by a blank line. Input is terminated by a value of zero (0) for n.

Output

For each test case, print one line telling whether arbitrage is possible or not in the format "Case case: Yes" respectively "Case case: No".

Sample Input

3USDollarBritishPoundFrenchFranc3USDollar 0.5 BritishPoundBritishPound 10.0 FrenchFrancFrenchFranc 0.21 USDollar3USDollarBritishPoundFrenchFranc6USDollar 0.5 BritishPoundUSDollar 4.9 FrenchFrancBritishPound 10.0 FrenchFrancBritishPound 1.99 USDollarFrenchFranc 0.09 BritishPoundFrenchFranc 0.19 USDollar0

Sample Output

Case 1: YesCase 2: No

思路：

对最短路的思想都深刻了解就能用求最短的思想求最长。这道题可以用好几种方法。

//Floyd

#include<iostream>#include<cstdio>#include<cstring>using namespace std;char c[32][40];double G[32][32];int n,m;void fory(){    for(int k=1;k<=n;k++)        for(int i=1;i<=n;i++)            for(int j=1;j<=n;j++)                if(G[i][k]*G[k][j]>G[i][j])                    G[i][j]=G[i][k]*G[k][j];}int main(){    char a[40],b[40];double rate;    int x,y;    int num=1;    while((scanf("%d",&n)!=EOF)&&n)    {        for(int i=1;i<=n;i++)        {            cin>>c[i];            G[i][i]=1;        }        scanf("%d",&m);        for(int i=1;i<=m;i++)        {            cin>>a>>rate>>b;            for(int j=1;j<=n;j++)            {                if(strcmp(c[j],a)==0)                    x=j;                if(strcmp(c[j],b)==0)                    y=j;            }            G[x][y]=rate;        }        fory();        int flag=0;        for(int i=1;i<=n;i++)        {            if(G[i][i]>1)            {                flag=1;                break;            }        }        if (flag)            cout<<"Case " << num++ << ": Yes" << endl;        else            cout << "Case " << num++ << ": No" << endl;    }    return 0;}