Kruskal
AcWing 859. Kruskal算法求最小生成树

#include <iostream>
#include <algorithm>

using namespace std;

const int N = 200010;//SE问题，也可能出现在这里，一般就是边界问题

int n, m;
int p[N];

struct Edge
{
    int a, b, w;

    bool operator< (const Edge &W)const
    {
        return w < W.w;
    }
}edges[N];

int find(int x)
{
    if (p[x] != x) p[x] = find(p[x]);
    return p[x];
}

int main()
{   
    scanf("%d%d", &n, &m);

    for (int i = 0; i < m; i++)
    {
        int a, b, w;
        scanf("%d%d%d", &a, &b, &w);
        edges[i] = {a, b, w};
    }

    //直接写kruscal, 不用把他单独包起来了
    sort(edges, edges + m);

    for (int i = 1; i <= n; i++) p[i] = i;

    int res = 0, cnt = 0;
    for (int i = 0; i < m; i++)
    {
        int a = edges[i].a, b = edges[i].b, w = edges[i].w;

        a = find(a), b = find(b);
        if (a != b)
        {
            p[a] = b;
            res += w;//最小生成树，所有树边的权重之和
            cnt++; //当前加了多少条边
        }
    }

    if (cnt < n - 1) puts("impossible");
    else printf("%d\n", res);

    return 0;
}