洛谷P2886牛继电器

传送门啦

倍增 $ Floyd $

注意结构体里二维数组不能开到 $ 2000 $

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#define Re register 
using namespace std;

inline int read(){
	char ch = getchar();
	int f = 1 , x = 0;
	while(ch > '9' || ch < '0') {if(ch == '-' ) f = -1 ; ch = getchar();}
	while(ch >= '0' && ch <= '9') {x = (x << 1) + (x << 3) + ch - '0' ; ch = getchar();}
	return x * f;
}

long long n,tot,m,s,t,x,y,z;
long long vis[20050],cnt;
long long w[2050][2050];

struct node{
	long long f[205][205];
	node() {
		memset(f , 0x3f , sizeof(f)) ;
	}
}ans ;

node mul(node a , node b) {
	node res ;
	for(Re int k = 1 ; k <= tot ; ++k)
		for(Re int i = 1 ; i <= tot ; ++i)
			for(Re int j = 1 ; j <= tot ; ++j)
				if(res.f[i][j] > a.f[i][k] + b.f[k][j])
					res.f[i][j] = a.f[i][k] + b.f[k][j];
	return res;
}

inline long long quick_power(long long k){
	node res ;
	for(Re int i = 1 ; i <= tot ; ++i) 
		res.f[i][i] = 0;
	while(k) {
		if(k & 1) res = mul(res , ans);
		ans = mul(ans , ans ) ;
		k >>= 1;
	}
	return res.f[vis[s]][vis[t]] ;
}

int main(){
	n = read();  m = read(); s = read(); t = read();
	for(Re int i = 1 ; i <= m ; ++i) {
		z = read() ; x = read() ; y = read() ;
		if(!vis[x])  vis[x] = ++tot;
		if(!vis[y])  vis[y] = ++tot;
		ans.f[vis[x]][vis[y]] = ans.f[vis[y]][vis[x]] = min(ans.f[vis[x]][vis[y]] , z) ;
	}
	printf("%lld\n" , quick_power(n));
	return 0;
}