以下来自kuangbin的最小树形图模版：
#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#include <math.h>
using namespace std;
/*
* 最小树形图
* 复杂度O(NM)
* 点从0开始
*/
const int INF = 100000000;
const int MAXN = 1010; //点数
const int MAXM = 40010;//边数
#define ll int
struct Edge{
int u,v;
ll cost;
}edge[MAXM]; //边从0开始
int pre[MAXN],id[MAXN],visit[MAXN];
ll in[MAXN];
ll zhuliu(int root,int n,int m,Edge edge[])//树根(注意是有向树，树根不能任意) 点数 边数 edge
{
int u,v;
ll res=0;
while(1)
{
for(int i = 0;i < n;i++)
in[i] = INF;
for(int i = 0;i < m;i++)
if(edge[i].u != edge[i].v && edge[i].cost < in[edge[i].v])
{
pre[edge[i].v] = edge[i].u;
in[edge[i].v] = edge[i].cost;
}
for(int i = 0;i < n;i++)
if(i != root && in[i] == INF)
return -1;//不存在最小树形图
int tn = 0;
memset(id,-1,sizeof(id));
memset(visit,-1,sizeof(visit));
in[root] = 0;
for(int i = 0;i < n;i++)
{
res += in[i];
v = i;
while( visit[v] != i && id[v] == -1 && v != root)
{
visit[v] = i;
v = pre[v];
}
if( v != root && id[v] == -1 )
{
for(int u = pre[v]; u != v ;u = pre[u])
id[u] = tn;
id[v] = tn++;
}
}
if(tn == 0)break;//没有有向环
for(int i = 0;i < n;i++)
if(id[i] == -1)
id[i] = tn++;
for(int i = 0;i < m;)
{
v = edge[i].v;
edge[i].u = id[edge[i].u];
edge[i].v = id[edge[i].v];
if(edge[i].u != edge[i].v)
edge[i++].cost -= in[v];
else
swap(edge[i],edge[--m]);
}
n = tn;
root = id[root];
}
return res;
}