#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<string>
#include<algorithm>
using namespace std;
int T,C,Ts,Te,a[10001],k,h[10001],dis[10001],minn,Rs,Re,Ci,x;
struct Edges
{
	int nxt,val,to;
}Edge[10001];
void add(int from,int to,int val)
{
	Edge[++k].to=to;
	Edge[k].val=val;
	Edge[k].nxt=h[from];
	h[from]=k;
}
int main()
{
	int i,j;
	cin>>T>>C>>Ts>>Te;
	memset(dis,0x7fffffff,sizeof(dis));
	dis[Ts]=0;
	for(i=1;i<=C;i++)
	{
		cin>>Rs>>Re>>Ci;
		add(Rs,Re,Ci);
		add(Re,Rs,Ci);
	}
	for(i=1;i<=T;i++)
	{
		minn=0x7fffffff;
		x=0;
		for(j=1;j<=T;j++)
			if(a[j]==0&&dis[j]<minn)
			{
				minn=dis[j];
				x=j;
			}
		if(x==0)
			break;
		a[x]=1;
		for(j=h[x];j;j=Edge[j].nxt)
			dis[Edge[j].to]=min(dis[Edge[j].to],dis[x]+Edge[j].val);
	}
	cout<<dis[Te];
	return 0;
}

题目