bzoj1690 [Usaco2007 Dec]奶牛的旅行（最优比率环，二分答案+spfa判负环）

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define inf 0x3f3f3f3f
#define N 1010
#define M 5010
#define eps 1e-5
inline int read(){
int x=0,f=1;char ch=getchar();
while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
while(ch>='0'&&ch<='9') x=x*10+ch-'0',ch=getchar();
return x*f;
}
int n,m,a
,h
;
double d
;
bool vis
;
struct edge{
int to,next,w;
}data[M];
inline bool spfa(int x,double mid){//判负环
vis[x]=1;
for(int i=h[x];i;i=data[i].next){
int y=data[i].to;double
1178c
val=data[i].w*mid-a[y];
if(d[x]+val<d[y]){
d[y]=d[x]+val;
if(vis[y]||spfa(y,mid)){vis[x]=0;return 1;}
}
}
vis[x]=0;return 0;
}
inline bool jud(double x){
memset(d,0,sizeof(d));
for(int i=1;i<=n;++i){
if(spfa(i,x)) return 1;
}
return 0;
}
int main(){
//  freopen("testdata.in","r",stdin);
n=read();m=read();
for(int i=1;i<=n;++i) a[i]=read();
for(int i=1;i<=m;++i){
int x=read(),y=read(),w=read();
data[i].to=y;data[i].next=h[x];h[x]=i;data[i].w=w;
}
double l=0.0,r=1000.0;
while(r-l>=eps){
double mid=(l+r)/2;
if(jud(mid)) l=mid;
else r=mid;
}
printf("%.2f\n",l);
return 0;
}