leetcode802+判断没有环的点，DFS_leetcode 802

https://leetcode.com/problems/find-eventual-safe-states/

//开始都是染色为0，2表示是安全无环，1表示有环有毒
class Solution {
public:
    bool dfs(int i, vector<int>&color, vector<vector<int>>&graph)
    {
        if(color[i]) return color[i] == 2;
        color[i] = 1; //开始染色为1，不确定是否为安全
        for(auto neighbor: graph[i]){
            if(color[neighbor]==1 || !dfs(neighbor, color, graph)){
                return false;
            }
        }
        color[i] = 2;
        return true;
    }
    vector<int> eventualSafeNodes(vector<vector<int>>& graph) {
        int len = graph.size();
        vector<int> color(len, 0);
        vector<int> ans;
        for(int i=0; i<len; i++){
            if(dfs(i, color, graph)){
                ans.push_back(i);
            }
        }
        return ans;
    }
};