LeetCode 323. Number of Connected Components in an Undirected Graph

#include <vector>
#include <unordered_map>
#include <queue>
#include <iostream>
using namespace std;
/*
  Given n nodes labeled from 0 to n-1 and a list of undirected edges, write a function
  to find the number of connected components in an undirected graph.
  For example:
  0
  |
  1 ---> 2    4--->3
  Given n = 5 and edges = [[0,1], [1, 2], [3, 4]] return 2.
*/
// it is pretty straightforward to come up BFS algorithm.
// But the first task is to change the pair vector into graph node-edges form.
// we can use hashMap to do that. unordered_map<int, vector<int> > hashMap.
void BFS(unordered_map<int, vector<int> >& hashMap, int node, unordered_map<int, bool>& visited) {
  queue<int> nodes;
  nodes.push(node);
  while(!nodes.empty()) {
    int tmp = nodes.front();
    nodes.pop();
    if(visited[tmp]) continue;  // if visited, skip it.
    visited[tmp] = true;
    auto iter = hashMap.find(tmp);
    vector<int> edges = iter->second;
    for(int k = 0; k < edges.size(); k++) {
      nodes.push(edges[k]);
    }
  }
}
int findNumOfConnectedComponent(int n, vector< vector<int> >& edges) {
  unordered_map<int, vector<int> > hashMap;
  unordered_map<int, bool> visited;
  int count = 0;
  for(int i = 0;  i < edges.size(); ++i) {
    hashMap[edges[i][0]].push_back(edges[i][1]);
    hashMap[edges[i][1]].push_back(edges[i][0]);
  }
  auto iter = hashMap.begin();
  while(iter != hashMap.end()) {
    visited[iter->first] = false;
    iter++;
  }
  auto it = visited.begin();
  while(it != visited.end()) {
    if(!(it->second)) {
      BFS(hashMap, it->first, visited);
      count++;
    }
    it++;
  }
  return count;
}
int main(void) {
  vector< vector<int> > edges {
  {0, 1},
  {1, 2},
  {3, 4},
  {5, 6},
  {7, 7}};
  cout << findNumOfConnectedComponent(8, edges) << endl;
}