bzoj 1486: [HNOI2009]最小圈 dfs求负环
1486: [HNOI2009]最小圈
Time Limit: 10 Sec Memory Limit: 64 MBSubmit: 1022 Solved: 487
[Submit][Status]
Description
![](http://www.lydsy.com/JudgeOnline/images/1486_1.jpg)
![](http://www.lydsy.com/JudgeOnline/images/1486_2.jpg)
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); }
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