深度優(yōu)先搜索，遍歷完一棵樹之后，選擇下一個(gè)沒有mark的點(diǎn)繼續(xù)搜索

class Solution {
public:
    void countNum(vector<vector<int>>& M, int ith, bool marked[]) {
        marked[ith] = true;
        for(int i = 0; i < M.size(); i++) {
            if(M[ith][i] == 1 && marked[i] == false) {
                countNum(M, i, marked);
            }
        }
    }
    int findCircleNum(vector<vector<int>>& M) {
        bool* markedNum = new bool[M.size()];
        int circleNum = 0;
        for(int i = 0; i < M.size(); i++)
            markedNum[i] = false;
        for(int i = 0; i < M.size(); i++) {
            if(!markedNum[i]) {
                countNum(M, i, markedNum);
                circleNum++;
            }
        }
        return circleNum;
    }
};