bzoj1486【HNOI2009】最小圈

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1486: [HNOI2009]最小圈

Time Limit: 10 Sec Memory Limit: 64 MB
Submit: 1778 Solved: 827
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Description

Input

Output

Sample Input

Sample Output

HINT

Source

01分数规划+二分答案+spfa判负环

#include<iostream>#include<cstdio>#include<cmath>#include<cstring>#include<cstdlib>#include<algorithm>#define F(i,j,n) for(int i=j;i<=n;i++)#define D(i,j,n) for(int i=j;i>=n;i--)#define ll long long#define maxn 3005#define maxm 10005#define inf 1000000000#define eps 1e-9using namespace std;struct edge{int next,to;double v,w;}e[maxm];int n,m,cnt,head[maxn];double dis[maxn];bool flag,mark[maxn];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;}inline void add_edge(int x,int y){e[++cnt].next=head[x];e[cnt].to=y;head[x]=cnt;}inline void spfa(int x){if (mark[x]){flag=true;return;}mark[x]=true;for(int i=head[x],y;i;i=e[i].next)if (dis[x]+e[i].v<dis[y=e[i].to]){dis[y]=dis[x]+e[i].v;spfa(y);if (flag) return;}mark[x]=false;}inline bool judge(){F(i,1,n) dis[i]=mark[i]=0;flag=false;F(i,1,n){spfa(i);if (flag) return true;}return false;}int main(){n=read();m=read();int x,y;double z,l=inf,r=-inf,mid;F(i,1,m){x=read();y=read();scanf("%lf",&z);add_edge(x,y);e[i].w=z;l=min(l,z);r=max(r,z);}while (r-l>=eps){mid=(l+r)/2;F(i,1,m) e[i].v=e[i].w-mid;if (judge()) r=mid;else l=mid;}printf("%.8lf\n",l);return 0;}

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