LeetCode - Friend Cycles

解法一：DFS

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

解法二： BFS

class Solution {
public:
    int findCircleNum(vector<vector<int>>& M) {
        int n = M.size();
        vector<bool> visited(n, false);
        queue<int> t;
        int res = 0;
        for(int i=0;i<n;i++){
            if(!visited[i]){
                t.push(i);
                while(!t.empty()){
                    int cur = t.front(); t.pop();
                    visited[cur] =true;
                    for(int j=0;j<n;j++){
                        if(M[cur][j]==1 && !visited[j]){
                            t.push(j);
                        }
                    }
                }
                res++;
            }
        }
        return res;
    }
};

解法三：Union Find

class Solution {
public:
    int findCircleNum(vector<vector<int>>& M) {
        int n = M.size(), 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 p1 = getroot(root,i);
                    int p2 = getroot(root,j);
                    if(p1!=p2){
                        res--;
                        root[p2] = p1;
                    }
                }
            }
        }
        return res;
    }
    int getroot(vector<int>& root, int x){
        while(x!=root[x]){
            x = root[x];
            root[x] = root[root[x]];
        }
        return x;
    }
};