题意： 从m个加油站里面选取1个站点，让他离居民区的最近的人最远，并且没有超出服务范围ds之内。如果有很多个最远的加油站，输出距离所有居民区距离平均值最小的那个。如果平均值还是一样，就输出按照顺序排列加油站编号最小的那个
tip：Dijkstra
#include <iostream>
#include <algorithm>
#include <string>
using namespace std;
const int inf = 1e9;
int n, m, k, ds, station;
int e[1020][1020], dis[1020];
bool visit[1020];
int main() {
fill(e[0], e[0] + 1020 * 1020, inf);
fill(dis, dis + 1020, inf);
scanf("%d%d%d%d", &n, &m, &k, &ds);
for(int i = 0; i < k; i++) {
int tempdis;
string s, t;
cin >> s >> t >> tempdis;
int a, b;
if(s[0] == 'G') {
s = s.substr(1);
a = n + stoi(s);
} else {
a = stoi(s);
}
if(t[0] == 'G') {
t = t.substr(1);
b = n + stoi(t);
} else {
b = stoi(t);
}
e[a][b] = e[b][a] = tempdis;
}
int ansid = -1;
double ansdis = -1, ansaver = inf;
for(int index = n + 1; index <= n + m; index++) {
double mindis = inf, aver = 0;
fill(dis, dis + 1020, inf);
fill(visit, visit + 1020, false);
dis[index] = 0;
for(int i = 0; i < n + m; i++) {
int u = -1, minn = inf;
for(int j = 1; j <= n + m; j++) {
if(visit[j] == false && dis[j] < minn) {
u = j;
minn = dis[j];
}
}
if(u == -1) break;
visit[u] = true;
for(int v = 1; v <= n + m; v++) {
if(visit[v] == false && dis[v] > dis[u] + e[u][v])
dis[v] = dis[u] + e[u][v];
}
}
for(int i = 1; i <= n; i++) {
if(dis[i] > ds) {
mindis = -1;
break;
}
if(dis[i] < mindis) mindis = dis[i];
aver += 1.0 * dis[i];
}
if(mindis == -1) continue;
aver = aver / n;
if(mindis > ansdis) {
ansid = index;
ansdis = mindis;
ansaver = aver;
} else if(mindis == ansdis && aver < ansaver) {
ansid = index;
ansaver = aver;
}
}
if(ansid == -1)
printf("No Solution");
else
printf("G%d\n%.1f %.1f", ansid - n, ansdis, ansaver);
return 0;
}
