求出各点与终点的距离 ，然后在求得的DAG 上做动规
#include<cstdio>
#include<cstring>
#include<queue>
#include<algorithm>
using namespace std;
struct my{
int v;
int next;
int dist;
};
struct headnode{
int dist;
int u;
bool operator <(const headnode& v)const{
return dist>v.dist;
}
};
typedef long long ll;
#define nil 0x7fffffff
const int maxn=100000+5;
int n,m;
int dp[maxn];
int adj[maxn+10];
bool vis[maxn+10];
int p[maxn+10];
int d[maxn+10];
my bian[maxn*2+10];
int fa;
void init(){
memset(adj,-1,sizeof(adj));
memset(bian,-1,sizeof(bian));
memset(p,0,sizeof(p));
memset(vis,false,sizeof(vis));
memset(dp,0,sizeof(dp));
fa=0;
}
void myinsert(int u,int v,int dist){
bian[++fa].v=v;
bian[fa].next=adj[u];
bian[fa].dist=dist;
adj[u]=fa;
}
void dijkstra(int x){
priority_queue<headnode>q;
headnode c;
for (int i=0;i<=n;i++) d[i]=nil;
d[x]=0;
c.u=x;
c.dist=0;
q.push(c);
while(!q.empty()){
c=q.top();
q.pop();
int u=c.u;
if(vis[u]) continue;
vis[u]=true;
for (int i=adj[u];i!=-1;i=bian[i].next){
my v=bian[i];
if(d[u]+v.dist<d[v.v]){
d[v.v]=d[u]+v.dist;
p[v.v]=u;
if(vis[v.v]) continue;
c.u=v.v;
c.dist=v.dist;
q.push(c);
}
}
vis[u]=false;
}
}
int DFS(int pos) {
if (pos == 2) return 1;
if (dp[pos] > 0) {
return dp[pos];
}
int ans = 0;
for (int i=adj[pos];i!=-1;i=bian[i].next) {
if (d[pos] > d[bian[i].v]) {
ans+=DFS(bian[i].v);
}
}
return dp[pos]=ans;
}
int main(){
int u,v,zhi;
while(scanf("%d",&n)&&n!=0){
scanf("%d",&m);
init();
for (int i=1;i<=m;i++){
scanf("%d%d%d",&u,&v,&zhi);
myinsert(u,v,zhi);
myinsert(v,u,zhi);
}
dijkstra(2);
printf("%d\n",DFS(1));
}
return 0;
}