hdu 1217 Arbitrage（spfa负圈）

Posted 2021-10-09 jpphy0

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问题

hdu 1217 Arbitrage - https://acm.hdu.edu.cn/showproblem.php?pid=1217

分析

• 从s出发回到s，大于1即可

代码

#include<bits/stdc++.h>
using namespace std;
const int MXN = 35;
int n, m, tot, head[MXN], inq[MXN];
double dis[MXN];
struct Edge{ int to, nxt; double dis;}edge[MXN*MXN];
map<string, int> cy;
void add(int u, int v, double d){
	Edge &e = edge[++tot];
	e.nxt = head[u], e.to = v, e.dis = d, head[u] = tot;
}
bool spfa(int s){
	queue<int> q;
	int now, to;
	for(int i = 1; i <= n; ++i) dis[i] = 0;
	memset(inq, 0, sizeof inq);
	dis[s] = 1.0, inq[s] = 1,  q.push(s);
	while(q.size()){
		now = q.front(), q.pop(), inq[now] = 0;
		for(int i = head[now]; ~i; i = edge[i].nxt){
			to = edge[i].to;
			if(dis[to] < dis[now] * edge[i].dis){
				if(to == s) return true;
				dis[to] = dis[now] * edge[i].dis;
				if(!inq[to]) q.push(to), inq[to] = 1;
			}
		}
	}
	return false;
}
int main(){
	int T = 0;
	bool ok;
	string s, t;
	double d;
	while(scanf("%d", &n) && n){
		if(!cy.empty()) cy.clear();
		for(int cnt = 0, i = 1; i <= n; ++i){
			cin >> s;
			if(cy.count(s) == 0) cy[s] = ++cnt;
		}
		memset(head, -1, sizeof head), tot = 0;
		scanf("%d", &m);
		for(int i = 1; i <= m; ++i){
			cin >> s >> d >> t;
			add(cy[s], cy[t], d);
		}
		for(int i = 1; i <= n; ++i) if(ok = spfa(i)) break; 
		printf("Case %d: %s\\n", ++T, ok ? "Yes" : "No");
	}
	return 0;
}