洛谷——P1195 口袋的天空

原题链接
这是一道简单的图论问题。首先，用n-k条边连成无环图，就有k棵树了，而题目的意思正是如此。利用Kruskal算法，就可以写了。

#include<iostream>
#include<cstring>
#include<algorithm>
#include<cmath>
using namespace std;

int un[1010];
int n, m, k;
struct Cl {
	int x, y, w;
}cl[10010];
bool cmp(Cl cl1, Cl cl2) {
	return cl1.w < cl2.w;
}
int find(int x) {
	if (x == un[x])
		return x;
	else return un[x] = find(un[x]);
}

void unity(int x, int y) {
	un[find(x)] = un[find((y))];
}

int main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	cin >> n >> m >> k;
	for (int i = 0; i < n; i++)
		un[i] = i;
	for (int i = 0; i < m; i++)
		cin >> cl[i].x >> cl[i].y >> cl[i].w;
	sort(cl, cl + m, cmp);

	int t = 0;
	int sum = 0;
	for (int i = 0; i < m; i++) {
		if (find(cl[i].x) != find(cl[i].y)) {
			unity(cl[i].x, cl[i].y);
			sum += cl[i].w;
			t++;
		}
		if (t == n - k) {
			cout << sum;
			return 0;
		}
	}
	cout << "No Answer";
	return 0;
}