按 kruskal 算法加边，直到连通块数量够了为止

#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1000;
const int MAXM = 10000;
int n, m, k;
// e[i] 存第 i 条边 {边权,{起点,终点}}
pair<int, pair<int, int>> e[MAXM + 5];
// 并查集
int fa[MAXN + 5];
int findFa(int x)
{
    if (x == fa[x])
        return x;
    return fa[x] = findFa(fa[x]);
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> m >> k;
    for (int i = 1; i <= m; i++)
    {
        // cin >> e[i].second.first >> e[i].second.second >> e[i].first;
        int u, v, w;
        cin >> u >> v >> w;
        e[i] = {w, {u, v}};
    }

    sort(e + 1, e + m + 1);
    int num = n; // 连通块数量
    int ans = 0; // 选择的边的权值之和
    for (int i = 1; i <= n; i++)
        fa[i] = i;
    for (int i = 1; num > k && i <= m; i++)
    {
        int u = e[i].second.first;
        int v = e[i].second.second;
        int w = e[i].first;
        int faU = findFa(u);
        int faV = findFa(v);
        if (faU == faV)
            continue;
        num--;
        fa[faU] = faV;
        ans += w;
    }
    if (num != k)
        cout << "No Answer";
    else
        cout << ans;
    return 0;
}