[Locked] Number of Connected Components in an Undirected Graph

Number of Connected Components in an Undirected Graph

Given n nodes labeled from 0 to n - 1 and a list of undirected edges (each edge is a pair of nodes), write a function to find the number of connected components in an undirected graph.

Example 1:

    0          3
    |          |
    1 --- 2    4

Given n = 5 and edges = [[0, 1], [1, 2], [3, 4]], return 2.

Example 2:

    0           4
    |           |
    1 --- 2 --- 3

Given n = 5 and edges = [[0, 1], [1, 2], [2, 3], [3, 4]], return 1.

分析：

　　类似于图像处理中找到物体轮廓的泛洪算法。在本题中，每个点扫一遍，每条边扫一遍，点和边都不重复遍历，复杂度为O(V + E)

代码：

int number(int n, vector<pair<int, int> > edges) {
    //Store edges with hash map for efficient access
    unordered_multimap<int, int> hash;
    for(auto e : edges) {
        hash.insert(e);
        hash.insert(make_pair(e.second, e.first));
    }
    //Flood fill algorithm
    vector<bool> visited(n, false);
    int number = 0;
    for(int i = 0; i < n; i++) {
        if(!visited[i]) {
            number++;
            visited[i] = true;
            queue<int> myq;
            myq.push(i);
            while(!myq.empty()) {
                auto range = hash.equal_range(myq.front());
                myq.pop();
                auto pos = range.first;
                while(pos != range.second) {
                    if(!visited[pos->second]) {
                        myq.push(pos->second);
                        visited[pos->second] = true;
                    }
                    pos++;
                }
            }
        }
    }
    return number;
}