bzoj 1486: [HNOI2009]最小圈 dfs求负环

1486: [HNOI2009]最小圈

Time Limit: 10 Sec  Memory Limit: 64 MB
Submit: 1022  Solved: 487
[Submit][Status]

Description

 

　　最开始写floyd求负环结果TLE了，改成dfs后速度变成原来的100+倍。反正还是比较神奇。

 

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
#define MAXN 4000
#define MAXE 50000
#define MAXV 10000
#define PROB "loop"
#define eps 1e-4
#define INF 0x3f3f3f3f3f3f3f3fLL
#define inf 1E1000
#ifdef unix
#define LL "%lld"
#else
#define LL "%I64d"
#endif
typedef long long qword;
typedef long double real;
struct edge
{
        int x,y;
        real z;
}el[MAXE];
struct Edge
{
        int np;
        real val;
        Edge *next;
}E[MAXE],*V[MAXV];
int tope=-1;
void addedge(int x,int y,real z)
{
        E[++tope].np=y;
        E[tope].val=z;
        E[tope].next=V[x];
        V[x]=&E[tope];
}
bool state[MAXN];
real dst[MAXN];
bool dfs(int now)
{
        Edge *ne;
        state[now]=true;
        for (ne=V[now];ne;ne=ne->next)
        {
                if (dst[ne->np]>dst[now]+ne->val)
                {
                        if (state[ne->np])return true;
                        dst[ne->np]=dst[now]+ne->val;
                        if (dfs(ne->np))return true;
                }
        }
        state[now]=false;
        return false;
}
int main()
{
      //  freopen(PROB".in","r",stdin);
        //freopen(PROB".out","w",stdout);
        freopen("input.txt","r",stdin);
        int n,m;
        scanf("%d%d",&n,&m);
        int i,j,k,x,y;
        real z;
        real l,r,mid;
        l=r=0;
        for (i=0;i<m;i++)
        {
                scanf("%d%d%Lf",&x,&y,&z);
                x--;y--;
                addedge(x,y,z);
        //        addedge(y,x,z);
                if(z<0)l+=z;
                else r+=z;
        }
        bool flag;
        while (l+1e-10<r)
        {
                mid=(l+r)/2;
                for (i=0;i<=n;i++)dst[i]=0,state[i]=false;;
                flag=false;
                for (i=0;i<=tope;i++)E[i].val-=mid;
                for (i=1;i<=n;i++)
                {
                        if (dfs(i))
                        {
                                flag=true;
                                break;
                        }
                }
                for(i=0;i<=tope;i++)E[i].val+=mid;
                if (flag)
                {
                        r=mid;
                }else
                {
                        l=mid;
                }
        }
        printf("%.8Lf\n",(real)r);
}