547. Friend Circles

最新推荐文章于 2020-06-15 09:07:48 发布

原创最新推荐文章于 2020-06-15 09:07:48 发布 · 469 阅读

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本文介绍了一种使用深度优先搜索(DFS)算法来计算一个班级中存在多少个互为直接或间接朋友的学生群体的方法。通过给出的学生间友谊关系矩阵，可以确定不同朋友圈的数量。

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 adirect 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 directfriends with each other, otherwise not. And you have to output the total number of friend circles among all the students.

Example 1:

Input: 
[[1,1,0],
 [1,1,0],
 [0,0,1]]
Output: 2
Explanation:The 0th and 1st students are direct friends, so they are in a friend circle. 
The 2nd student himself is in a friend circle. So return 2.

Example 2:

Input: 
[[1,1,0],
 [1,1,1],
 [0,1,1]]
Output: 1
Explanation:The 0th and 1st students are direct friends, the 1st and 2nd students are direct friends, 
so the 0th and 2nd students are indirect friends. All of them are in the same friend circle, so return 1.

Note:

N is in range [1,200].
M[i][i] = 1 for all students.

If M[i][j] = 1, then M[j][i] = 1.

思路：DFS

class Solution {
public:
    int findCircleNum(vector<vector<int>>& M) {
        int count = 0;
        vector<bool> visited(M.size(), false);
        for (int i = 0; i < M.size(); i++) {
            if (visited[i] == false) {
                visited[i] = true;
                count++;
                helper(M, visited, i);
            }
        }
        return count;
    }
private:
    void helper(vector<vector<int>>& M, vector<bool>& visited, int cur) {
        for (int i = 0; i < M.size(); i++) {
            if ((M[cur][i] == 1) && (visited[i] == false)) {
                visited[i] = true;
                helper(M, visited, i);
            }
        }
    }
};

My github-leetcode: https://github.com/QiangL-DUT/LeetCode