[BZOJ 2180]最小直径生成树

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#define maxn 210
using namespace std;
const int inf = 0x7fffffff / 2;

int n, m;
int f[maxn][maxn], d[maxn][maxn], p[maxn][maxn];

void MST(){
	//Floyd
	for(int k = 1; k <= n; k ++)
	    for(int i = 1; i <= n; i ++)
	        for(int j = 1; j <= n; j ++)
	            if(i != j && j != k && k != i)
	                f[i][j] = min(f[i][j], f[i][k] + f[k][j]);
	                
	//getrank
	for(int i = 1; i <= n; i ++){
		for(int j = 1; j <= n; j ++)
		    p[i][j] = j;
		for(int j = 1; j <= n; j ++)
			for(int k = j + 1; k <= n; k ++)
			    if(f[i][p[i][j]] > f[i][p[i][k]])
			        swap(p[i][j], p[i][k]);
	}
	
	//main
	int ans = inf;
	for(int u = 1; u <= n; u ++){
		for(int v = 1; v <= n; v ++){
			if(d[u][v] != inf){
				ans = min(ans, min(f[u][p[u]
] << 1, f[v][p[v]
] << 1));
				int t = n;
				for(int k = n - 1; k >= 1; k --){
					if(f[v][p[u][t]] < f[v][p[u][k]]){
						ans = min(ans, f[u][p[u][k]] + f[v][p[u][t]] + d[u][v]);
						t = k;
					}
				}
			}
		}
	}
	printf("%d\n", ans);
}

int main(){
	scanf("%d%d", &n, &m);
	for(int i = 1; i <= n; i ++)
		for(int j = 1; j <= n; j ++)
			f[i][j] = d[i][j] = inf;
	for(int i = 1; i <= n; i ++)
	    f[i][i] = d[i][i] = 0;
	int u, v, dis;
	for(int i = 1; i <= m; i ++){
		scanf("%d%d%d", &u, &v, &dis);
		f[u][v] = f[v][u] = d[u][v] = d[v][u] = dis;
	}
	MST();
    return 0;
}