leetcode--547. Friend Circles

There are N students in a class. Some of them are friends, while some are not. Their friendship is transitive in nature. For example, if A is a direct friend of B, and B is a direct friend of C, then A is an indirect friend of C. And we defined a friend circle is a group of students who are direct or indirect friends.

Given a N*N matrix M representing the friend relationship between students in the class. If M[i][j] = 1, then the ith and jth students are direct friends with each other, otherwise not. And you have to output the total number of friend circles among all the students.

为一个统计共有多少个连通区的问题。如果A与B连通，B与C连通，则A与C连通。此题运用深度优先搜索DFS。

最外层利用一个for循环，检查每一个节点是否被访问过，如果没有被访问过，则将其送入深度优先搜索的函数，寻找它的连通点。并且将连通区数量加一。

深度优先搜索函数，利用递归，将每两两节点关系进行检查。

class Solution {
public:
    
    int len;
    int group[200] = {0};  // record if this node has been visited

    void dfs(vector>& M, int visited) {
        for (int i = 0; i>& M) {
        len = M[0].size();
        int max = 0;
        for (int i = 0; i