【Cerc2012】Farm and factory

题面
题解
#include<cstdio>
#include<cstring>
#include<iostream>
#include<queue>
#include<algorithm>
#include<vector>
#define ri register int
#define N 100050
#define LL long double
#include<cmath>
using namespace std;

int n,m;
vector<int> to[N];
vector<LL> len[N];
LL dis[N][2];
bool vis[N];
LL x[N],y[N];

struct node {
  int x;
  LL d;
  bool operator < (const node &rhs) const {
    return d>rhs.d;
  }
};

void add_edge(int x,int y,LL z) {
  to[x].push_back(y); len[x].push_back(z);
  to[y].push_back(x); len[y].push_back(z);
}

void dij(int s) {
  priority_queue<node> q;
  for (ri i=1;i<=n;i++) dis[i][s]=1e18;
  memset(vis,0,sizeof(vis));
  dis[s+1][s]=0;
  q.push((node){s+1,dis[s+1][s]});
  while (!q.empty()) {
    int x=q.top().x; q.pop();
    if (vis[x]) continue;
    vis[x]=1;
    for (ri i=0;i<to[x].size();i++) {
      int y=to[x][i];
      if (dis[y][s]>dis[x][s]+len[x][i]) {
        dis[y][s]=dis[x][s]+len[x][i];
        q.push((node){y,dis[y][s]});
      }
    }
  }
}

int main() {
  int T,a,b;
  LL c;
  scanf("%d",&T);
  while (T--) {
    scanf("%d %d",&n,&m);
    for (ri i=1;i<=n;i++) to[i].clear(),len[i].clear();
    for (ri i=1;i<=m;i++) {
      scanf("%d %d %Lf",&a,&b,&c);
      add_edge(a,b,c);
    }
    dij(0);
    dij(1);
    for (ri i=1;i<=n;i++) x[i]=dis[i][0]+dis[i][1],y[i]=dis[i][0]-dis[i][1];
    sort(x+1,x+n+1); sort(y+1,y+n+1);
    LL sum=0;
    for (ri i=1;i<=n;i++) {
      sum+=fabs(x[i]-x[(n+1)/2])+fabs(y[i]-y[(n+1)/2]);
    }
    sum/=2*n;
    printf("%.8Lf
", sum);
  }
  return 0;
}