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某些無良出題人可能會卡bfs找負環,所以要用dfs
核心代碼(以jzoj5173為例):
#include<bits/stdc++.h>
using namespace std;
#define eps 5e-5
int n,m,v[10010],nxt[10010],h[1010],s,t,ct[1010],inq[1010],ec;
double w[10010],dis[1010];
void add(int x,int y,double z){v[++ec]=y;w[ec]=z;nxt[ec]=h[x];h[x]=ec;}
int dfs(int x){
inq[x]=1;
for(int i=h[x];i;i=nxt[i])
if(dis[v[i]]>dis[x]+w[i]){
dis[v[i]]=dis[x]+w[i];
if(inq[v[i]])return 1;
else if(dfs(v[i]))return 1;
}
inq[x]=0;
return 0;
}
int spfa(){
memset(inq,0,sizeof(inq));
memset(dis,0,sizeof(dis));
for(int i=1;i<=n;i++)
if(!inq[i]&&dfs(i))return 1;
return 0;
}
int main(){
scanf("%d%d",&n,&m);
while(m--){
int a,b;double c;
scanf("%d%d%lf",&a,&b,&c);
add(a,b,c);
}
double l=-1,r=10000010;
while(fabs(r-l)>eps){
double mid=(l+r)*0.5;
for(int i=1;i<=ec;i++)
w[i]-=mid;
if(spfa())r=mid-eps;
else l=mid+eps;
for(int i=1;i<=ec;i++)
w[i]+=mid;
}
if(fabs(l-10000010)<eps)printf("PaPaFish is laying egg!");
else printf("%.2lf",l);
}