poj2240--Arbitrage--Bellmand-Ford算法

 

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
//好开心，我终于知道原来我以前一直在用的，说不出名字其实本质上就是Bellman-Ford算法#include <iostream>#include <cstdio>#include <cstring>#include <string>#include <queue>using namespace std;#define maxn 38#define inf 0x3f3f3f3fstring A[maxn];double map[maxn][maxn];double d[maxn];//d[maxn]用来存换成该货币的目前最大钱int main(){int n,m,time=0;while(scanf("%d",&n)!=EOF&&n){time++;bool flag=false;memset(map,0,sizeof(map));for(int i=1;i<=n;i++){cin>>A[i];}scanf("%d",&m);string a,b;double c;for(int i=1;i<=m;i++){cin>>a>>c>>b;int u,v;for(int j=1;j<=n;j++){if(A[j]==a){u=j;}if(A[j]==b){v=j;}}map[u][v]=c;}//地图存好for(int i=1;i<=n;i++)//以每种货币为起点枚举一遍{for(int j=1;j<=n;j++){if(i==j){d[j]=1;}else d[j]=0;}queue <int> q;q.push(i);while(!q.empty()){int p=q.front();q.pop();for(int j=1;j<=n;j++){if(map[p][j]&&d[p]*map[p][j]>d[j]){d[j]=d[p]*map[p][j];q.push(j);if(j==i){flag=true;goto L;}}}}}L:printf("Case %d: %s\n",time,flag?"Yes":"No");}return 0;}