“蔚来杯“2022牛客暑期多校训练营2 Link with Game Glitch (spfa找正负环)

#include<bits/stdc++.h>

using namespace std;

#define X first
#define Y second
#define eps 1e-10
#define gcd __gcd
#define pb push_back
#define PI acos(-1.0)
#define lowbit(x) (x)&(-x)
#define bug printf("!!!!!\n");
#define mem(x,y) memset(x,y,sizeof(x))

typedef long long LL;
typedef long double LD;
typedef pair<int,int> pii;
typedef unsigned long long uLL;

const int maxn = 1e5+2;
const int INF  = 1<<30;
const int mod  = 1e9+7;

struct edge {
    
  int v;
  double w;
};

int n,m;
vector<edge> e[maxn];
int cnt[maxn], vis[maxn];
double dis[maxn];

bool spfa(int n,double x) {
    
  // memset(dis, 63, sizeof(dis));
  queue<int> q;
  x=log(x);
  // cout<<x<<endl;
  for(int i=1;i<=n;i++) dis[i]=-1e9,cnt[i]=0,q.push(i),vis[i]=1;  
    // cout<<w+x<<endl;
  while (!q.empty()) {
    
    int u = q.front();
    q.pop(), vis[u] = 0;
    for (auto ed : e[u]) {
    
      int v = ed.v;
      double w = ed.w;
      // cout<<w+x<<endl;
      if (dis[v] < dis[u] + w + x) {
    
        dis[v] = dis[u] + w + x;
        cnt[v] = cnt[u] + 1;
        if (cnt[v] >= n) {
    
            // cout<<v<<" "<<"false"<<endl;
            return false;
        }
        if (!vis[v]) q.push(v), vis[v] = 1;
      }
    }
  }
  return true;
}

void solve(){
    
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++) e[0].push_back({
    i,0});
    for(int i=1;i<=m;i++){
    
        int a,b,c,d;
        scanf("%d%d%d%d",&a,&b,&c,&d);
        e[b].push_back({
    d,log(1.0*c/a)});
        // cout<<log(1.0*c/a)<<endl;
    }
    // cout<<spfa(n,0.6)<<endl;
    double l=0,r=1;
    while(abs(r-l)>=eps){
    
        double mid=(l+r)/2;
        if(spfa(n,mid)) l=mid;
        else r=mid;
    }
    printf("%0.6lf\n",l);
    return;
}

int main()
{
    
// freopen("in.txt","r",stdin);
// freopen("out.txt","w",stdout);
// ios::sync_with_stdio(false);
    int t = 1;
    //scanf("%d",&t);
    while(t--){
    
    // printf("Case %d: ",cas++);
        solve();
    }
    return 0;
}