CareerCup

We need a HashTable<Directory>. Then traverse the tree in preOrder. For each directory d do. 1) push d in HT and if d is already exist then return false. 2) recursively do for each children node of d. 3)if all return true then remove d from HT and return true.

public boolean check(Directory d){
   if(HT.get(d))
       return false;
   else{
     for(int i=0;i<d.subdir.length;i++){
         if(check(d.subdir[i]) == false)
            return false;
     }//endfor
     HT.remove(d);
     return true;
   }
}

I guess there is some sort of symbolic link from child to an ancestor,
or else I don't know how we can make a child subdir point to an ancestor dir.

If thats the understanding, then the prob boils down to, given a directed graph,
find a back edge upon dfs (basically detecting a cycle).

perform a DFS and keep adding the directories in a hash table with their corresponding level number. For eg. root directory will have level number 0 and all sub-directories of root directory have level number 1 and so on.
Now if the directory pointed by soft link is present in hash table and the level number is less then the current level number, then it is pointing to one of it's ancestor

May be just a DFS traversal will do, given the structure is stored as graphs

Create a BST or a sorted list of addresses only. If there's a clash, you've the directory that links to one of the parents.

DFS travel a graph. using colors to indicate the traversal status.