bzoj 1486: [HNOI2009]最小圈

Posted 2020-07-02 Swm_sxt

二分答案再判负环

 

#include<cstdio>
#include<algorithm>
using namespace std;
int read_p,read_ca;
inline int read(){
    read_p=0;read_ca=getchar();
    while(read_ca<‘0‘||read_ca>‘9‘) read_ca=getchar();
    while(read_ca>=‘0‘&&read_ca<=‘9‘) read_p=read_p*10+read_ca-48,read_ca=getchar();
    return read_p;
}
struct na{
    int y,ne;
    double z;
}b[10001];
int n,m,l[3001],r[3001],ru[3001],num=0;
int x,y,z,i,j,k;
double dis[3001];
const double INF=1e9;
double mi=1e7,ma=0;
bool bo[3001];
inline void add(){
    x=read();y=read();z=read();
    num++;
    if (!l[x]) l[x]=num;else b[r[x]].ne=num;
    b[num].y=y;b[num].z=(double)z;r[x]=num;
    if (b[num].z>ma) ma=b[num].z;
    if (b[num].z<mi) mi=b[num].z;
}
inline bool spfa(int p,double x){
    bo[p]=ru[p]=1;
    for (int j=l[p];j;j=b[j].ne)
    if (ru[b[j].y]&&dis[p]+b[j].z-x<=dis[b[j].y]) return 1;
    for (int j=l[p];j;j=b[j].ne)
    if (dis[b[j].y]>dis[p]+b[j].z-x){
        dis[b[j].y]=dis[p]+b[j].z-x;
        if (spfa(b[j].y,x)) return 1;
    }
    ru[p]=0;
    return 0;
}
inline bool run(double x){
    register int i;
    for (i=1;i<=n;i++) dis[i]=0,bo[i]=ru[i]=0;
    for (i=1;i<=n;i++)
    if (!bo[i])
    if (spfa(i,x)) return 1;
    return 0;
}
int main(){
    n=read();m=read();
    while (m--) add();
    double l=mi,r=ma,mid;
    while(r-l>=1e-9){
        mid=(l+r)/((double)(2.0));
        if (run(mid)) r=mid;else l=mid;
    }
    printf("%.8lf",l);
    return 0;
}

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