[bzoj1706] [usaco2007 Nov]relays 奶牛接力跑

#include<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>
#include<cstdlib>
#define ll long long
#define ull unsigned long long
#define d double
using namespace std;
const int maxn=1023,inf=1002333333;
struct mat{int f[202][202];}f,c;
int id[1023];
int i,j,k,n,m,cnt,st,ed;

int ra,fh;char rx;
inline int read(){
rx=getchar(),ra=0,fh=1;
while((rx<'0'||rx>'9')&&rx!='-')rx=getchar();
if(rx=='-')fh=-1,rx=getchar();
while(rx>='0'&&rx<='9')ra*=10,ra+=rx-48,rx=getchar();return ra*fh;
}
inline int min(int a,int b){return a<b?a:b;}
inline mat run(mat &a,mat &b){
register int i,j,k;
for(i=1;i<=cnt;i++)for(j=1;j<=cnt;j++)
for(c.f[i][j]=inf,k=1;k<=cnt;k++)
c.f[i][j]=min(c.f[i][j],a.f[i][k]+b.f[k][j]);
return c;
}
int main(){
n=read(),m=read(),st=read(),ed=read();int x;
memset(f.f,60,sizeof(f.f));
for(i=1;i<=m;i++){
x=read(),j=read(),k=read();
if(!id[j])id[j]=++cnt;if(!id[k])id[k]=++cnt;
j=id[j],k=id[k];
f.f[j][k]=min(f.f[j][k],x),f.f[k][j]=min(f.f[k][j],x);
}
mat g=f;n--;
while(n){
if(n&1)g=run(g,f);
n>>=1;
if(n)f=run(f,f);
}
printf("%d\n",g.f[id[st]][id[ed]]);
}