POJ

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.
用map将字符串当做点，Bellman-Ford求正权回路……

#include<iostream>#include<queue>#include<math.h>#include<stdio.h>#include<string>#include<map>using namespace std;#define N 1000+5#define LL long long int#define pow(a) ((a)*(a))#define INF 0x3f3f3f3f#define mem(arr,a) memset(arr,a,sizeof(arr))map<string, int>maps;struct edge{    int from;    int to;    double rate;};edge es[N];int V, E;double d[N];int counts = 1;int bellman(int s){    mem(d, 0);    d[s] = 1;    int cnt = 0;    while (1){        cnt++;        bool update = false;;        for (int i = 0; i < E; i++){            if (d[es[i].to]<d[es[i].from] * es[i].rate){                d[es[i].to] = d[es[i].from] * es[i].rate;                update = true;            }        }        if (!update)return false;        if (cnt == V)return true;    }    return false;}int main(){    while (cin >> V)    {        if (V == 0)break;        for (int i = 1; i <= V; i++){            string s; cin >> s;            maps[s] = i;        }        cin >> E;        for (int i = 0; i < E; i++){            string a, b;            double rate;            cin >> a;            cin >> rate;            cin >> b;            es[i].rate = rate;            es[i].from = maps[a];            es[i].to = maps[b];        }        printf("Case %d: ", counts++);        if (bellman(1))cout << "Yes" << endl;        else cout << "No" << endl;    }    return 0;}