345.牛站

思路

变种Flyod+快速幂思想（倍增），还需要提前做离散化。

AC code

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#include <iostream>
#include <cstring>
#include <algorithm>
#include <map>

using namespace std;

const int N = 210;
int g[N][N];
int res[N][N];
int k, n, m, s, e;

void mul(int c[][N], int a[][N], int b[][N]){
    static int temp[N][N];
    memset(temp, 0x3f, sizeof(temp));
    for(int k = 1; k <= n; k++){
        for(int i = 1; i <= n; i++){
            for(int j = 1; j <= n; j++){
                temp[i][j] = min(temp[i][j], a[i][k] + b[k][j]);
            }
        }
    }
    memcpy(c, temp, sizeof(temp));
}

void qmi(){
    memset(res, 0x3f, sizeof(res));
    for(int i = 1; i <= n; i++) res[i][i] = 0;
    
    while(k){
        if(k & 1) mul(res, res, g);
        mul(g, g, g);
        k >>= 1;
    }
}

int main(){
    cin >> k >> m >> s >> e;
    
    memset(g, 0x3f, sizeof(g));
    map<int, int> mp;
    if(!mp.count(s)) mp[s] = ++n;
    if(!mp.count(e)) mp[e] = ++n;
    s = mp[s], e = mp[e];
    
    while (m -- ){
        int a, b, c;
        cin >> c >> a >> b;
        if(!mp.count(a)) mp[a] = ++n;
        if(!mp.count(b)) mp[b] = ++n;
        a = mp[a], b = mp[b];
        g[a][b] = g[b][a] = min(g[a][b], c);
    }
    
    qmi();
    
    cout << res[s][e] << endl;
    return 0;
}