Important Problems on Trees and Graphs for Interviews and Self Assessment
I will list down some problems on the topic of trees and graphs that are commonly asked in interviews. Please revise topics on tress such as Binary Trees, Binary Search Tree (BST), Balancing a Tree, Full and Complete Trees, Binary Tree Traversals (recursive), Tries, Depth First Search (DFS), Breadth First Search (BFS).
Problems
Trees
 Check if a binary tree is a Binary Search Tree (BST)

Check if a given binary tree is balanced.
 Assumption: Heights of two subtrees of any node should never differ by more than one, to consider a tree as balanced.
 Solution: http://javajee.com/solutioncheckifabinarytreeisbalancedinontime
 Create a binary search tree with minimal height from a sorted increasing order array.

Find difference between sum of nodes at odd levels and sum of nodes at even levels of a binary tree.
 Difference = (Sum of nodes at odd levels)  (Sum of nodes at even levels)
 Assumption: Root is at level 1 and left and right children of root are at level 2
 Solution (recursive): http://javajee.com/solutiondifferencebetweensumofnodesatoddlevel...

Given a Binary Search Tree (BST), create a linked list of nodes for every level.
 If you have a tree with depth D, you will have D linked lists.
 Solution (recursive): http://javajee.com/solutioncreatealinkedlistofallnodesateachle...
 Solution (iterative): http://javajee.com/solutioncreatealinkedlistofallnodesateachle...

Given a binary tree with an integer data element; find all paths whose sum of data nodes will be equal to a given value
 Path can start or end anywhere in the tree
 Solution: http://javajee.com/solutionfindallpathswhosesumofdatanodeswill...

Check if a binary tree (T2) is a sub tree of another tree (T1)
 T1 has millions of nodes and T2 has hundreds of nodes

Find the inorder successor (next node) of a given node in a Binary Search Tree (BST)?
 Each node has a link to its parent

Find the first common ancestor of two nodes in a binary tree
 Avoid storing additional nodes in any data structure
 The binary tree might not be a Binary Search Tree (BST)

Traverse the tree in spiral (zig zag) form and print nodes.

Consider below tree levels:
 1
 2 3
 4 5 6 7
 8 9
 Result should be: 1 3 2 4 5 6 7 9 8

Consider below tree levels:
Graphs
 Find out if there is a route between two nodes in a directed graph.
Refer to individual problems under corresponding problem category for solutions.