题意

题目链接

Sol

暴力01分数规划可过

标算应该是这个

#include<bits/stdc++.h>

#define Pair pair<int, double>

#define MP(x, y) make_pair(x, y)

#define fi first

#define se second

//#define int long long

#define LL long long

#define Fin(x) {freopen(#x".in","r",stdin);}

#define Fout(x) {freopen(#x".out","w",stdout);}

using namespace std;

const int MAXN = 3001, mod = 998244353, INF = 2e9 + 10;

const double eps = 1e-9;

template <typename A, typename B> inline bool chmin(A &a, B b){if(a > b) {a = b; return 1;} return 0;}

template <typename A, typename B> inline bool chmax(A &a, B b){if(a < b) {a = b; return 1;} return 0;}

template <typename A, typename B> inline LL add(A x, B y) {if(x + y < 0) return x + y + mod; return x + y >= mod ? x + y - mod : x + y;}

template <typename A, typename B> inline void add2(A &x, B y) {if(x + y < 0) x = x + y + mod; else x = (x + y >= mod ? x + y - mod : x + y);}

template <typename A, typename B> inline LL mul(A x, B y) {return 1ll * x * y % mod;}

template <typename A, typename B> inline void mul2(A &x, B y) {x = (1ll * x * y % mod + mod) % mod;}

template <typename A> inline void debug(A a){cout << a << '\n';}

template <typename A> inline LL sqr(A x){return 1ll * x * x;}

inline int read() {

    char c = getchar(); int x = 0, f = 1;

    while(c < '0' || c > '9') {if(c == '-') f = -1; c = getchar();}

    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();

    return x * f;

}

int N, M;

vector<Pair> v[MAXN];

double a[MAXN], dis[MAXN];

int vis[MAXN], times[MAXN], can[MAXN];

bool SPFA(int S,  double k) {

	queue<int> q; q.push(S);

	for(int i = 1; i <= N; i++) vis[i] = 0, times[i] = 0, dis[i] = INF;

	dis[S] = 0;

	times[S]++;

	while(!q.empty()) {

		int p = q.front(); q.pop(); vis[p] = 0;

		can[p] = 1;

		for(auto &sta : v[p]) {

			int to = sta.fi; double w = sta.se;

			if(chmin(dis[to], dis[p] + w - k)) {

				if(!vis[to]) q.push(to), vis[to] = 1, times[to]++;

				if(times[to] > 50) return 1;

			}

		}

	}

	return 0;

}

bool check(double val) {

	memset(can, 0, sizeof(can));

	for(int i = 1; i <= N; i++)

		if(!can[i] && SPFA(i, val)) return 1;

	return 0;

}

signed main() {

	//Fin(a);

	N = read(); M = read();

	for(int i = 1; i <= M; i++) {

		int x = read(), y = read(); double z; scanf("%lf", &z);

		v[x].push_back({y, z});

	}

	double l = -1e7 - 10, r = 1e7 + 10;

	while(r - l > eps) {

		double mid = (l + r) / 2;

		if(check(mid)) r = mid;

		else l = mid;

	}

	printf("%.8lf", l);

	return 0;

}

/*

10 3

aaaabbbbab

7 7 3 9 10 6 7 6 6 1

2 6 2

1 3 1

2 9 1

*/