OJ-Problems-Source/HDOJ/3143_autoAC.cpp

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#include <cstdio>
#include <cstring>
#include <queue>
#include <algorithm>
using namespace std;
const int N = 100+5;
int head[N],cnt,n,e,p,d;
#define pc 160
#define eps 1e-8
#define INF 0x3f3f3f3f
double dist1[N],dist2[N];
bool out[N];
struct edges
{
int u,cost,next;
edges(){}
edges(int a,int b,int c):u(a),cost(b),next(c){}
}edge[N*N];
void init(){
memset(head,-1,sizeof(head));
memset(out,0,sizeof(out));
cnt = 0;
}
void add_edge(int a,int b,int c){
edge[cnt] = edges(b,c,head[a]);
head[a] = cnt++;
}
bool inQue[N];
queue<int>que;
void spfa(double dist[],int src)
{
for(int i = 1; i <= n; ++i) dist[i] = INF,inQue[i] = 0;
while(!que.empty()) que.pop();
que.push(src);
dist[src] = 0;
inQue[src] = true;
while(!que.empty())
{
int u = que.front();que.pop();
for(int i = head[u]; i + 1 ; i =edge[i].next)
{
int v = edge[i].u;
int val = edge[i].cost;
if(src == p && v == d) continue;
if(dist[u] + val < dist[v])
{
dist[v] = dist[u] + val;
if(!inQue[v]) que.push(v),inQue[v] = true;
}
}
inQue[u] = false;
}
}
queue<int>qq;
bool has[N];
bool bfs(double vi)
{
while(!qq.empty()) qq.pop();
qq.push(p);
for(int i = 1; i <= n; ++i) has[i] = 0;
while(!qq.empty())
{
int u = qq.front();
if(out[u]) return true;
qq.pop();
for(int i = head[u]; i + 1; i = edge[i].next)
{
int v = edge[i].u;
if(has[v]||dist1[v]*pc>=dist2[v]*vi) continue;
qq.push(v);
has[v] = 1;
}
}
return false;
}
int main()
{
int m;
while(scanf("%d%d%d",&n,&m,&e)!=EOF)
{
init();
for(int i = 0,x,y,z; i < m; ++i)
{
scanf("%d%d%d",&x,&y,&z);
add_edge(x,y,z);
add_edge(y,x,z);
}
for(int i = 0; i < e; ++i) scanf("%d",&p),out[p]=1;
scanf("%d%d",&p,&d);
spfa(dist1,p);
spfa(dist2,d);
double left = 0,right = INF,ans = -1;
while(left+eps<right)
{
double mid = (right+left)/2;
if(bfs(mid)) right= mid,ans = mid;
else left = mid;
}
if(ans < 0)printf("IMPOSSIBLE\n");
else printf("%.9f\n",ans);
}
return 0;
}