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

class Solution {public:    vector
     
       visited;    vector
      
        mem;    // -1 unknown, 0 unsafe, 1 safe.    int n;    vector
       
         eventualSafeNodes(vector
        
         
          >& graph) {        n = graph.size();        mem = vector
          
           (n, -1); visited = vector
           
            (n, false); for (int i = 0; i < n; i++) dfs(graph, i); vector
            
              res; for (int i = 0; i < n; i++) if (mem[i] == 1) res.push_back(i); return res; } bool dfs(vector
             
              
               >& graph, int i) { // check if i is evetually safe if (mem[i] != -1) return mem[i] == 1; bool res = true; if (visited[i]) {　　// A loop res = false; } else { visited[i] = true; for (auto j : graph[i]) { if (!dfs(graph, j)) { res = false; break; } } // visited[i] = false; // This line is optional, since we've cached the result for i in mem. } mem[i] = res; return res; }};