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  • BZOJ 1690 奶牛的旅行

    分数规划。

    #include<iostream>
    #include<cstdio>
    #include<cstring>
    #include<algorithm>
    #define maxv 1050
    #define maxe 10050
    #define inf 2000000000
    #define eps 1e-3
    using namespace std;
    int n,m,val[maxv],nume=1,g[maxv],x,y,z,flag=0;
    struct edge
    {
        int v,c,nxt;
        double w;
    }e[maxe];
    double dis[maxv];
    bool vis[maxv];
    void addedge(int u,int v,int c)
    {
        e[++nume].v=v;e[nume].c=c;
        e[nume].nxt=g[u];g[u]=nume;
    }
    void rebuild(double x)
    {
        for (int i=2;i<=nume;i++)
            e[i].w=e[i].c*x-val[e[i].v];
    }
    void spfa(int x)
    {
        vis[x]=true;
        for (int i=g[x];i;i=e[i].nxt)
        {
            int v=e[i].v;
            if (dis[v]>dis[x]+e[i].w)
            {
                if (vis[v]) {flag=1;break;}
                else {dis[v]=dis[x]+e[i].w;spfa(v);}
            }
        }
        vis[x]=false;
    }
    bool check(double x)
    {
        rebuild(x);
        for (int i=1;i<=n;i++) dis[i]=inf;
        dis[1]=-val[1];flag=0;spfa(1);
        return flag;
    }
    void binary_search()
    {
        double l=0,r=inf,ans;
        while (r-l>=eps)
        {
            double mid=(l+r)/2;
            if (check(mid)) {ans=mid;l=mid;}
            else r=mid; 
        }
        printf("%.2lf
    ",ans);
    }
    int main()
    {
        scanf("%d%d",&n,&m);
        for (int i=1;i<=n;i++) scanf("%d",&val[i]);
        for (int i=1;i<=m;i++)
        {
            scanf("%d%d%d",&x,&y,&z);
            addedge(x,y,z);
        }
        binary_search();
        return 0;
    }
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  • 原文地址:https://www.cnblogs.com/ziliuziliu/p/6050977.html
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