bzoj 1486: [HNOI2009]最小圈 （01分数规划+深搜版spfa）

1486: [HNOI2009]最小圈

Time Limit: 10 Sec   Memory Limit: 64 MB
Submit: 2224   Solved: 1046
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Description

 

Input

Output

Sample Input

4 5
1 2 5
2 3 5
3 1 5
2 4 3
4 1 3

Sample Output

3.66666667

HINT

Source

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题解：01分数规划+深搜版spfa

#include
#include
#include
#include
#include
#include
#define N 20003
#define eps 1e-9
#define inf 1e8
using namespace std;
int n,m,can[N],tot;
int point[N],next[N],v[N];
double len[N],dis[N],mid;
bool pd;
void add(int x,int y,double z)
{
	tot++; next[tot]=point[x]; point[x]=tot; v[tot]=y; len[tot]=z;
}
void spfa(int x)
{
	can[x]=1;
	for (int i=point[x];i;i=next[i])
	 if (dis[v[i]]>dis[x]+len[i]-mid) {
	 	dis[v[i]]=dis[x]+len[i]-mid;
	 	if (can[v[i]]==1) {
	 		pd=true;
	 		return;
		 }
		spfa(v[i]);
		if (pd) return;
	 }
	can[x]=0;
}
bool check(double x)
{
	memset(can,0,sizeof(can));
	memset(dis,0,sizeof(dis));
	for (int i=1;i<=n;i++) {
		pd=false;
		spfa(i);
		if (pd) return true;
	} 
    return false;
}
int main()
{
	freopen("a.in","r",stdin);
	scanf("%d%d",&n,&m);
	double l=0,r=0;
	for (int i=1;i<=m;i++) {
		int x,y; double z;
		scanf("%d%d%lf",&x,&y,&z);
		add(x,y,z); 
	}
	double ans=inf; l=-inf; r=inf;
	while (r-l>=eps) {
		mid=(l+r)/2;
		if (check(mid)) ans=min(ans,mid),r=mid-eps;
		else l=mid+eps;
	}
	printf("%.8lf\n",ans);
}