并查集

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

kruscal

#include<iostream>
#include<algorithm>

using namespace std;

const int N = 1e5 + 10;

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() {
    cin >> n >> m;

    for(int i = 0; i < m; i ++) {
        int a, b, w;
        cin >> a >> b >> w;
        edges[i] = {a, b, w};
    }

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

    sort(edges, edges + m);

    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;
            cnt ++ ;
            res += w;
        }
    }

    if(cnt < n - 1) puts("impossible");
    else cout << res << endl;

    return 0;
}