POJ 3621

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int maxn = 1023;
const double eps = 1e-3;
typedef long long LL;
struct node {
	int v, next;
	double w;
	int t;
	node() {
	}
	node(int v, int next, double w, int t) :
			v(v), next(next), w(w), t(t) {
	}
} edge[maxn << 3];
int head[maxn], e;
double f[maxn];
void add_edge(int u, int v, int w) {
	edge[e] = node(v, head[u], 0, w);
	head[u] = e++;
}
bool inq[maxn];
int cnt[maxn];
double dis[maxn];
int q[maxn], front, rear;
bool spfa(int s, int n) {
	int i, u, v;
	for (i = 1; i <= n; ++i)
		dis[i] = -1e10, cnt[i] = 0;
	dis[1] = 0;
	q[rear++] = s;
	cnt[s]++;
	while (front != rear) {
		u = q[front++];
		front &= maxn;
		inq[u] = false;
		for (i = head[u]; ~i; i = edge[i].next) {
			v = edge[i].v;
			if (dis[u] + edge[i].w > dis[v]) {
				dis[v] = dis[u] + edge[i].w;
				if (!inq[v]) {
					inq[v] = true;
					q[rear++] = v;
					rear &= maxn;
					if ((++cnt[v]) >= n)
						return true;
				}
			}
		}
	}
	return false;
}
int main() {
	int n, m, u, v, i, w;
	scanf("%d%d", &n, &m);
	double ma;
	for (i = 1; i <= n; ++i) {
		scanf("%lf", f + i);
		head[i] = -1;
		ma = max(ma, f[i]);
	}
	e = 0;
	for (i = 0; i < m; ++i) {
		scanf("%d%d%d", &u, &v, &w);
		add_edge(u, v, w);
	}
	double l = 0, r = ma, mid;
	while (r - l > eps) {
		mid = (l + r) * 0.5;
		for (i = 0; i < m; ++i) {
			edge[i].w = f[edge[i].v] - mid * edge[i].t;
		}
		if (spfa(1, n))
			l = mid;
		else
			r = mid;
	}
	printf("%.2f\n", mid);
	return 0;
}