This was asked in Google Interview.

Given a list of parent-child relationships, build a tree out of it.

Example Input:

80 19
50 20
20 15
80 16
50 80
20 17

You should return the following tree:

         50
        /  \
      20    80
     / \    / \
    15 17  19 16

PS: This doesn't have to be a binary tree. You need to return a root node.