下面Prim算法程序中，横线处应该填入的是（ ）。 endl;

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int prim(vector<vector<int>> & graph, int n) {
    vector<int> key(n, INT_MAX);
    vector<int> parent(n, -1);
    key[0] = 0;
    for (int i = 0; i < n; i++) {
        int u = min_element(key.begin(), key.end()) - key.begin();
        if (key[u] == INT_MAX)
            break;
        for (int v = 0; v < n; v++) {
            if (__________) { // 在此处填入选项
                key[v] = graph[u][v];
                parent[v] = u;
            }
        }
    }
    int sum = 0;
    for (int i = 0; i < n; i++) {
        if (parent[i] != -1) {
            cout << "Edge: " << parent[i] << " - " << i << " Weight: " << key[i] <<

            sum += key[i];
        }
    }
    return sum;
}
int main() {
    int n, m;
    cin >> n >> m;
    vector<vector<int>> graph(n, vector<int>(n, 0));
    for (int i = 0; i < m; i++) {
        int u, v, w;
        cin >> u >> v >> w;
        graph[u][v] = w;
        graph[v][u] = w;
    }
    int result = prim(graph, n);
    cout << "Total weight of the minimum spanning tree: " << result << endl;
    return 0;
}