BZOJ 1486: [HNOI2009]最小圈 01分数规划+SPFA判环

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这里写图片描述

Sample Input
4 5

1 2 5

2 3 5

3 1 5

2 4 3

4 1 3
Sample Output
3.66666667

解法：01分数规划+SPFA判环

///BZOJ 1486#include <bits/stdc++.h>using namespace std;const int maxn = 3005;const int maxm = 30005;int n, m, edgecnt, head[maxn];double dis[maxn];bool vis[maxn];struct edge{    int v, nxt;    double w;    edge(){}    edge(int v, int nxt, double w):v(v),nxt(nxt),w(w){}}E[maxm];int u[maxm], v[maxm];double w[maxm];void init(){    memset(head, -1, sizeof(head));    edgecnt=0;}void add(int u, int v, double w){    E[edgecnt].v = v, E[edgecnt].nxt = head[u], E[edgecnt].w = w, head[u] = edgecnt++;}void build(double mid){    init();    for(int i=1; i<=m; i++) add(u[i],v[i],w[i]-mid);}int flag;void spfa(int x){    vis[x]=1;    for(int i=head[x];i+1;i=E[i].nxt){        int v=E[i].v;        if(dis[x]+E[i].w<dis[v]){            if(vis[v]){                flag=1;            }            else{                dis[v]=dis[x]+E[i].w;                spfa(v);            }        }    }    vis[x]=0;}bool check(double mid){    build(mid);    memset(vis,0,sizeof(vis));    memset(dis,0,sizeof(dis));    flag=0;    for(int i=1; i<=n; i++){        spfa(i);        if(flag) return 1;    }    return 0;}int main(){    scanf("%d%d", &n,&m);    for(int i=1; i<=m; i++){        scanf("%d%d%lf",&u[i],&v[i],&w[i]);    }    double l = -10000000, r = 10000000;    for(int i=0; i<100; i++){        double mid=(l+r)/2;        if(check(mid)) r=mid;        else l=mid;    }    printf("%.8f\n", l);    return 0;}

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