[leetcode]547. Friend Circles

[leetcode]547. Friend Circles

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Analysis

ummmmmmmmmmmmmmmmmmmmmmmmm~—— [ummmm~]

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.
经典的并查集~

Implement

class Solution {
public:
    int findCircleNum(vector<vector<int>>& M) {
        int n = M.size();
        int res = n;
        vector<int> root(n);
        for(int i=0; i<n; i++)
            root[i] = i;
        for(int i=0; i<n; i++){
            for(int j=i+1; j<n; j++){
                if(M[i][j] == 1){
                    int ri = getRoot(root, i);
                    int rj = getRoot(root, j);
                    if(ri != rj){
                        res--;
                        root[ri] = rj;
                    }
                }
            }
        }
        return res;
    }
    int getRoot(vector<int>& root, int id){
        while(id != root[id]){
            root[id] = root[root[id]];
            id = root[id];
        }
        return id;
    }
};