http://www.luogu.org/problem/show?pid=1576
把每次扣除的百分数z换成1-z%的样子作边权，即每次要乘的数值，找个乘积最长路最后用100除这个最长路即可。Dijkstra和SPFA应该都可以，然而我只会打SPFA。。。还要注意是双向边。。。
#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<queue>
#define INF 0x3f3f3f3f
using namespace std;
const int maxn=2010;
int head[maxn*maxn],k=1,n,m,a,b;
double dis[maxn];
bool vis[maxn];
queue<int> q;
struct edge{int v,next;double w;
}e[maxn*maxn];
void addedge(int u,int v,double w){e[k].v=v;e[k].w=w;e[k].next=head[u];head[u]=k++;}
double spfa(int s)
{memset(vis,0,sizeof(vis));memset(dis,0,sizeof(dis));queue<int> q;dis[s]=1;vis[s]=1;q.push(s);while(!q.empty()) {int t=q.front();q.pop();vis[t]=0;for(int i=head[t];i;i=e[i].next){int v=e[i].v;if(dis[v]<dis[t]*e[i].w){dis[v]=dis[t]*e[i].w;if(!vis[v]){vis[v]=1;q.push(v);}}}} return 100/dis[b];
}
int main()
{scanf("%d%d",&n,&m);for(int i=1;i<=m;i++){int x,y,z;scanf("%d%d%d",&x,&y,&z);addedge(x,y,1-1.0*z/100);addedge(y,x,1-1.0*z/100);}scanf("%d%d",&a,&b);printf("%.8lf",spfa(a)*1.0);return 0;
}