#6510. 「雅礼集训 2018 Day8」A [最小树形图]

建一个超级源点，建新边，然后就可以对于这个超级源点跑最小树形图了

// powered by c++11
// by Isaunoya
#include <bits/stdc++.h>
#define rep(i, x, y) for (register int i = (x); i <= (y); ++i)
#define Rep(i, x, y) for (register int i = (x); i >= (y); --i)
using namespace std;
using db = double;
using ll = long long;
using uint = unsigned int;
#define Tp template
using pii = pair<int, int>;
#define fir first
#define sec second
Tp<class T> void cmax(T& x, const T& y) {
  if (x < y) x = y;
}
Tp<class T> void cmin(T& x, const T& y) {
  if (x > y) x = y;
}
#define all(v) v.begin(), v.end()
#define sz(v) ((int)v.size())
#define pb emplace_back
Tp<class T> void sort(vector<T>& v) { sort(all(v)); }
Tp<class T> void reverse(vector<T>& v) { reverse(all(v)); }
Tp<class T> void unique(vector<T>& v) { sort(all(v)), v.erase(unique(all(v)), v.end()); }
const int SZ = 1 << 23 | 233;
struct FILEIN {
  char qwq[SZ], *S = qwq, *T = qwq, ch;
#ifdef __WIN64
#define GETC getchar
#else
  char GETC() { return (S == T) && (T = (S = qwq) + fread(qwq, 1, SZ, stdin), S == T) ? EOF : *S++; }
#endif
  FILEIN& operator>>(char& c) {
    while (isspace(c = GETC()))
      ;
    return *this;
  }
  FILEIN& operator>>(string& s) {
    while (isspace(ch = GETC()))
      ;
    s = ch;
    while (!isspace(ch = GETC())) s += ch;
    return *this;
  }
  Tp<class T> void read(T& x) {
    bool sign = 0;
    while ((ch = GETC()) < 48) sign ^= (ch == 45);
    x = (ch ^ 48);
    while ((ch = GETC()) > 47) x = (x << 1) + (x << 3) + (ch ^ 48);
    x = sign ? -x : x;
  }
  FILEIN& operator>>(int& x) { return read(x), *this; }
  FILEIN& operator>>(ll& x) { return read(x), *this; }
} in;
struct FILEOUT {
  const static int LIMIT = 1 << 22;
  char quq[SZ], ST[233];
  int sz, O;
  ~FILEOUT() { flush(); }
  void flush() {
    fwrite(quq, 1, O, stdout);
    fflush(stdout);
    O = 0;
  }
  FILEOUT& operator<<(char c) { return quq[O++] = c, *this; }
  FILEOUT& operator<<(string str) {
    if (O > LIMIT) flush();
    for (char c : str) quq[O++] = c;
    return *this;
  }
  Tp<class T> void write(T x) {
    if (O > LIMIT) flush();
    if (x < 0) {
      quq[O++] = 45;
      x = -x;
    }
    do {
      ST[++sz] = x % 10 ^ 48;
      x /= 10;
    } while (x);
    while (sz) quq[O++] = ST[sz--];
  }
  FILEOUT& operator<<(int x) { return write(x), *this; }
  FILEOUT& operator<<(ll x) { return write(x), *this; }
} out;
#define int long long

int n, m;
const int maxn = 5e2 + 25;
const int maxm = maxn * maxn;
int u[maxm], v[maxm], w[maxm];
int from[maxn], mnin[maxn], col[maxn];
bool vis[maxn], ins[maxn];
int nn, mm;
void dfs(int x) {
  if (!x || !from[x] == -1) return;
  vis[x] = ins[x] = 1;
  if (!vis[from[x]])
    dfs(from[x]);
  else if (ins[from[x]]) {
    col[x] = ++nn;
    for (int i = from[x]; i != x; i = from[i]) col[i] = nn;
  }
  ins[x] = 0;
  if (!col[x]) col[x] = ++nn;
}

const int inf = 1e15;
signed main() {
  // code begin.
  in >> n >> m;
  rep(i, 1, m) in >> u[i] >> v[i] >> w[i];
  rep(i, 1, n) { u[++m] = 0, v[m] = i, w[m] = inf; }
  int ans = 0;
  while (1) {
    rep(i, 1, n) mnin[i] = inf << 1, from[i] = -1;
    rep(i, 1, m) if (w[i] < mnin[v[i]]) { mnin[v[i]] = w[i], from[v[i]] = u[i]; }
    rep(i, 0, n) vis[i] = col[i] = 0;
    nn = mm = 0;
    rep(i, 1, n) {
      ans += mnin[i];
      if (!vis[i]) dfs(i);
    }
    if (n == nn) break;
    rep(i, 1, m) {
      if (col[u[i]] != col[v[i]]) {
        w[++mm] = w[i] - mnin[v[i]];
        u[mm] = col[u[i]], v[mm] = col[v[i]];
      }
    }
    n = nn, m = mm;
  }
  if (ans >= 2 * inf)
    out << -1 << '
';
  else
    out << ans - inf << '
';
  return 0;
  // code end.
}