how to find a node that is known to everyone but doesn't know anyone?
This can be done with the help of an auxiliary stack and elimination technique.
Let us call the node we are looking for the special node. We can make certain conclusions by examining the relation of any two nodes as following :


If node A knows node B, then A is surely not the special node and B may be the special node.
If node A doesn't know node B, then B is surely not special node and A may be the special node.
We repeat above two steps till we are left with only one node.
Examine the last node if it doesn't know anyone and is known to everyone.


Algorithm: (uses auxiliary stack)


Push all the nodes on the stack.
Pop off top two nodes from the stack, discard one node based on the observations made.
Push the remained node onto stack.
Repeat step 2 and 3 until only one node remains in the stack.
Check the remained node is the one we are looking for.


Why do we need the last step? 

Because we have examined all the nodes relative to only one node which is adjacent to it on the stack but we don't know its relation with other nodes. So we need to ensure that the potential candidate for the special node is actually the special node.

This will require $3(n-1)$ comparisons in total. So the complexity is $O(n)$.