15.Kruskal算法求最小生成树

 

 

#include <bits/stdc++.h>
using namespace std;
const int N = 200010;
struct edge {
    int a, b, w;
    friend bool operator < (edge e1, edge e2) {
        return e1.w < e2.w;
    }
} edges[N];
int n, m;
int p[N]; //并查集里面的p
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 = a;
        edges[i].b = b;
        edges[i].w = w;
    }
    sort(edges, edges + m); //1.将所有边按照权重从小到大排序
    for (int i = 1; i <= n; i++) { //初始化并查集
        p[i] = i;
    }
    int res = 0, cnt = 0;
    //权重之和，边的数量
    for (int i = 0; i < m; i++) { //2.从小到大枚举每条边
        int a = edges[i].a, b = edges[i].b, w = edges[i].w;
        a = find(a), b = find(b);
        if (a != b) { //并查集，如果不连通
            p[a] = b; //加进来
            res += w;
            cnt++;
        }
    }
    if (cnt < n - 1) { //不连通
        cout << "impossible" << endl;
    } else {
        cout << res << endl;
    }
    return 0;
}