下面最小生成树的Kruskal算法程序中，横线处应该填入的是（ ）。

1#include <iostream>2#include <vector>3#include <algorithm>4using namespace std;5struct Edge {6   int u, v, weight;7   bool operator <(const Edge & other) const {8      return weight < other.weight;9   }10};11int findParent(int vertex, vector<int> & parent) {12   if (parent[vertex] == -1)13      return vertex;14   return parent[vertex] = findParent(parent[vertex], parent);15}16int main() {17   int n, m;18   cin >> n >> m; // n: 顶点数, m: 边数19   vector<Edge> edges(m);20   vector<int> parent(n, -1);21   int totalWeight = 0;22   for (int i = 0; i < m; i++)23      cin >> edges[i].u >> edges[i].v >> edges[i].weight;24   sort(edges.begin(), edges.end());25 26   for (const auto & edge : edges) {27      int uParent = findParent(edge.u, parent);28      int vParent = findParent(edge.v, parent);29      if (__________) { // 在此处填入选项30         parent[uParent] = vParent;31         totalWeight += edge.weight;32      }33   }34}