bzoj1486

二分答案 若存在负圈则说明答案可以更优 dfs版spfa判负圈

#include<bits/stdc++.h>
#define clr(a,x) memset(a,x,sizeof(a))
#define rep(i,l,r) for(int i=l;i<r;i++)
typedef long long ll;
using namespace std;
int read()
{
    char c=getchar();
    int ans1=0,f=1;
    while(!isdigit(c)){
        if(c=='-') f=-1;
        c=getchar();
    }
    while(isdigit(c)){
        ans1=ans1*10+c-'0';
        c=getchar();
    }
    return ans1*f;
}
struct edge{
    int to;double d,v;
};
const double esp=1e-9,inf=1e7;
const int maxn=3005;
int n,m;
double d[maxn]; 
bool p[maxn],flag;
vector<edge>e[maxn];
void dfs(int a)
{
    p[a]=1;
    rep(i,0,e[a].size()){
        if(d[a]+e[a][i].v<d[e[a][i].to])
        {
            if(!p[e[a][i].to]){
                d[e[a][i].to]=d[a]+e[a][i].v;
                dfs(e[a][i].to);
            }else flag=1;
            if(flag){
                p[a]=0;
                return;
            }
        }
    }
    p[a]=0;
}
bool ok(double a)
{
    clr(p,0),clr(d,0),flag=0;
    rep(i,1,n+1){
        rep(j,0,e[i].size()){
            e[i][j].v=e[i][j].d-a;
        }
    }
    rep(i,1,n+1){
        dfs(i);
        if(flag) return 1;
    }
    return 0;
}
double find(double l,double r)
{
    if(r-l<=esp) return l;
    double mid=(l+r)/2;
    return ok(mid)?find(l,mid):find(mid,r);
}
int main()
{
    n=read(),m=read();
    rep(i,0,m){
        int from=read();
        edge ed;
        ed.to=read();
        scanf("%lf",&ed.d);
        e[from].push_back(ed);
    }
    printf("%.8lf
",find(-inf,inf));
    return 0;
}

1486: [HNOI2009]最小圈

Time Limit: 10 Sec  Memory Limit: 64 MB
Submit: 1448  Solved: 680
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