解法：

初始化二维汇率图

经过 Floyd-Warshall 算法后，二维数组会被更新为所有货币对之间的最大汇率路径。如果存在 g[i][i] > 1，则表示存在套汇。

#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
using namespace std;
const int N = 1e3;
double g[N][N];
int main() {
	int n, m, cnt = 1;
	while (cin >> n, n) {
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < n; j++) {
				g[i][j] = 0;
			}
		}
		string s1, s2;
		map<string, int> mp;
		for (int i = 0; i < n; i++) {
			cin >> s1;
			mp[s1] = i;
		}
		cin >> m;
		for (int i = 0; i < m; i++) {
			double x;
			cin >> s1 >> x >> s2;
			g[mp[s1]][mp[s2]] = x;
		}
		for (int k = 0; k < n; k++) {
			for (int i = 0; i < n; i++) {
				for (int j = 0; j < n; j++) {
					g[i][j] = max(g[i][j], g[i][k] * g[k][j]);
				}
			}
		}
		int flag = 0;
		for (int i = 0; i < n; i++) {
			if (g[i][i] > 1) {
				flag = 1;
				break;
			}
		}
		cout << "Case " << cnt << " " << (flag ? "Yes" : "No") << endl;
		cnt++;
	}
	return 0;
}