【POJ 3164】Command Network(最小树形图)

1.​​题目链接​​​。最小树形图问题：最小树形图就是有向图的最小生成树，当然生成的是一颗有向树。具体可见：最小树形图。

2.这个题就是最小树形图的裸题了，偷了大佬的板子。

//#include<bits/stdc++.h>
#include<iostream>
#include<cstring>
#include<cmath>
#include<cstdio>
#include<stdio.h>
#define INF 1e9
#define N 10001
#define LL long long
using namespace std;
struct Node 
{
  double x, y;
}node[N];
struct Edge {
  int x, y;
  double w;
}edge[N];
int vis[N];
int id[N];
int pre[N];
double in[N];
double zhuLiu(int root, int n, int m) {
  double res = 0;
  while (true) {

    for (int i = 0; i < n; i++)
      in[i] = INF;

    for (int i = 0; i < m; i++) {
      int x = edge[i].x;
      int y = edge[i].y;
      if (edge[i].w < in[y] && x != y) {
        pre[y] = x;
        in[y] = edge[i].w;
      }
    }
    for (int i = 0; i < n; i++) {
      if (i == root)
        continue;
      if (in[i] == INF)
        return -1;
    }

    int cnt = 0;
    in[root] = 0;
    memset(id, -1, sizeof(id));
    memset(vis, -1, sizeof(vis));
    for (int i = 0; i < n; i++) {
      res += in[i];

      int y = i;
      while (vis[y] != i && id[y] == -1 && y != root) {
        vis[y] = i;
        y = pre[y];
      }

      if (y != root && id[y] == -1) {
        for (int x = pre[y]; x != y; x = pre[x])
          id[x] = cnt;
        id[y] = cnt++;
      }
    }
    if (cnt == 0)
      break;
    for (int i = 0; i < n; i++)
      if (id[i] == -1)
        id[i] = cnt++;

    for (int i = 0; i < m; i++) {
      int x = edge[i].x;
      int y = edge[i].y;
      edge[i].x = id[x];
      edge[i].y = id[y];

      if (id[x] != id[y])
        edge[i].w -= in[y];
    }

    n = cnt;
    root = id[root];
  }
  return res;
}
double calculate(Node a, Node b) {
  return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
}
int main() {
  int n, m;
  while (scanf("%d%d", &n, &m) != EOF && (n + m)) 
  {
    for (int i = 0; i < n; i++)
      scanf("%lf%lf", &node[i].x, &node[i].y);
    for (int i = 0; i < m; i++) {
      scanf("%d%d", &edge[i].x, &edge[i].y);
      //点的编号是从[0,n-1]
      edge[i].x--;
      edge[i].y--;
      if (edge[i].x != edge[i].y)
        edge[i].w = calculate(node[edge[i].x], node[edge[i].y]);
      else
        edge[i].w = INF;//如果存在自环，就赋值为INF
    }
    double res = zhuLiu(0, n, m);//最小生成树的边权
    if (res == -1)
      printf("poor snoopy\n");
    else
      printf("%.2f\n", res);
  }
  return 0;
}