洛谷 P2212 【[USACO14MAR]Watering the Fields S】

一道最小生成树模板题，这里用的Kruskal算法，把每两点就加一条边，跑一遍最小生成树即可。

#include 
using namespace std;
struct node{
	int l , r , w;
};
node e[4000010];
int n , maxx , tot , now , ans;
int fa[2010] , a[2010] , b[2010];
int find(int x){
	if(x == fa[x]) return x;
	return fa[x] = find(fa[x]);
}
bool cmp(node x , node y){
	return x.w < y.w;
}
int work(int i , int j){
	return (a[i] - a[j]) * (a[i] - a[j]) + (b[i] - b[j]) * (b[i] - b[j]);
}
int main(){
	cin >> n >> maxx;
	for(int i = 1; i <= n; i++){
		fa[i] = i;
		cin >> a[i] >> b[i];
	}
	for(int i = 1; i <= n; i++)
		for(int j = 1; j < i; j++){	//只用输入一半
			int x = work(i , j);
			if(x < maxx) continue;
			tot++;
			e[tot].l = i;
			e[tot].r = j;
			e[tot].w = x;
		}
	sort(e + 1 , e + tot + 1 , cmp);
	for(int i = 1; i <= tot; i++){
		if(now == n - 1) break;
		int x = find(e[i].l) , y = find(e[i].r);
		if(x == y) continue;
		fa[x] = y;
		now++;
		ans += e[i].w;
	}
	if(now != n - 1) cout << -1;	//边数不够 
	else cout << ans;
	return 0;
}