bzoj 1690: [Usaco2007 Dec]奶牛的旅行 spfa+二分答案

题意

分析

代码

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;

const int N=1005;
const double eps=1e-3;

int n,m,cnt,last
,ins
,val
;
double dis
;
struct edge{int to,next,w;double v;}e[N*10];

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;
}

void addedge(int u,int v,int w)
{
e[++cnt].to=v;e[cnt].w=w;e[cnt].next=last[u];last[u]=cnt;
}

bool dfs(int x)
{
ins[x]=1;
for (int i=last[x];i;i=e[i].next)
if (dis[x]+e[i].v<dis[e[i].to])
{
if (ins[e[i].to]) return 1;
dis[e[i].to]=dis[x]+e[i].v;
if (dfs(e[i].to)) return 1;
}
ins[x]=0;
return 0;
}

bool check(double mid)
{
for (int i=1;i<=cnt;i++) e[i].v=(double)e[i].w*mid-val[e[i].to];
for (int j=1;j<=n;j++) dis[j]=0,ins[j]=0;
for (int i=1;i<=n;i++) if (dfs(i)) return 1;
return 0;
}

int main()
{
n=read();m=read();
for (int i=1;i<=n;i++) val[i]=read();
for (int i=1;i<=m;i++)
{
int x=read(),y=read(),w=read();
addedge(x,y,w);
}
double l=0,r=10000;
while (r-l>eps)
{
double mid=(l+r)/2;
if (check(mid)) l=mid;
else r=mid;
}
printf("%.2lf",l);
return 0;
}