1486: [HNOI2009]最小圈
Time Limit: 10 Sec Memory Limit: 64 MBSubmit: 979 Solved: 473
[Submit][Status]
Description
题解：
湖南题为什么出个这么裸的判负环，dfs的SPFA即可，精度把握好。
代码：
1 #include<cstdio> 2 #include<cstdlib> 3 #include<cmath> 4 #include<cstring> 5 #include<algorithm> 6 #include<iostream> 7 #include<vector> 8 #include<map> 9 #include<set> 10 #include<queue> 11 #include<string> 12 #define inf 1000000000 13 #define maxn 3005 14 #define maxm 10005 15 #define eps 1e-9 16 #define ll long long 17 #define pa pair<int,int> 18 using namespace std; 19 inline int read() 20 { 21 int x=0,f=1;char ch=getchar(); 22 while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();} 23 while(ch>='0'&&ch<='9'){x=10*x+ch-'0';ch=getchar();} 24 return x*f; 25 } 26 struct edgs{int go,next;double w,w0;}e[maxm]; 27 double d[maxn]; 28 int n,m,tot,v[maxn],head[maxn]; 29 bool mark[maxn],flag; 30 void insert(int x,int y,double z) 31 { 32 e[++tot].go=y;e[tot].w0=z;e[tot].next=head[x];head[x]=tot; 33 } 34 void spfa(int x) 35 { 36 if(mark[x]){flag=1;return;} 37 mark[x]=1; 38 for(int i=head[x],y;i;i=e[i].next) 39 if(d[x]+e[i].w<d[y=e[i].go]) 40 { 41 d[y]=d[x]+e[i].w; 42 spfa(y); 43 if(flag)return; 44 } 45 mark[x]=0; 46 } 47 bool check() 48 { 49 for(int i=1;i<=n;i++)d[i]=mark[i]=0; 50 flag=0; 51 for(int i=1;i<=n;i++) 52 { 53 spfa(i); 54 if(flag)return 1; 55 } 56 return 0; 57 } 58 int main() 59 { 60 freopen("input.txt","r",stdin); 61 freopen("output.txt","w",stdout); 62 n=read();m=read(); 63 int x,y;double z,l=inf,r=-inf,mid; 64 while(m--) 65 { 66 x=read();y=read();scanf("%lf",&z);insert(x,y,z); 67 l=min(l,z); 68 r=max(r,z); 69 } 70 while(r-l>=eps) 71 { 72 mid=(l+r)/2; 73 for(int i=1;i<=tot;i++)e[i].w=e[i].w0-mid; 74 if(check())r=mid;else l=mid; 75 } 76 printf("%.8lf ",l); 77 return 0; 78 }