bzoj1486: [HNOI2009]最小圈

#include<cstdio>
#include<algorithm>
#include<cstring>
#define eps 1e-10
using namespace std;
const int maxn = 10000 + 10;
const int maxm = 200000 + 10;
int g[maxn],v[maxm],next[maxm],eid;
double w[maxm],dist[maxn],e[maxm],c;
bool vis[maxn];
int n,m;

void addedge(int a,int b,double c) {
    v[eid]=b; w[eid]=c; next[eid]=g[a]; g[a]=eid++;
}

void build() {
    memset(g,-1,sizeof(g));
    scanf("%d%d",&n,&m);
    for(int i=1,a,b;i<=m;i++) {
        scanf("%d%d%lf",&a,&b,&c);
        addedge(a,b,c);
    }
}

bool dfs(int u) {
    vis[u]=1;
    for(int i=g[u];~i;i=next[i]) if(dist[v[i]]>dist[u]+e[i]) {
        if(vis[v[i]]) return true;
        dist[v[i]]=dist[u]+e[i];
        if(dfs(v[i])) return true;
    }
    vis[u]=0;
    return false;
}

bool calc() {
    memset(dist,0,sizeof(dist));
    memset(vis,0,sizeof(vis));    
    for(int i=1;i<=n;i++) if(dfs(i)) return 1;
    return 0;
}

void solve() {
    double l = -1e9,r=1e9,mid;
    while(r-l>=eps) {
        mid=(l+r)/2;
        for(int u=1;u<=n;u++)
        for(int i=g[u];~i;i=next[i]) 
            e[i]=w[i]+mid;
        if(calc()) l=mid;
        else r=mid;
    }
    printf("%.8lf\n",-l);
}

int main() {
    build();
    solve();    
    return 0;    
}