Reverse Breadth First traversal in C#

Question

Anyone has a ready implementation of the Reverse Breadth First traversal algorithm in C#?

By Reverse Breadth First traversal , I mean instead of searching a tree starting from a common node, I want to search the tree from the bottom and gradually converged to a common node.

Let's see the below figure, this is the output of a Breadth First traversal :

In my reverse breadth first traversal , 9,10,11 and 12 will be the first few nodes found ( the order of them are not important as they are all first order). 5, 6, 7 and 8 are the second few nodes found, and so on. 1 would be the last node found.

Any ideas or pointers?

Edit: Change "Breadth First Search" to "Breadth First traversal" to clarify the question

Answer 1

Run a normal BFS from rootNode and let depth[i] = linked list of nodes with depth i. So for your example you'll have:

depth[1] = {1}, depth[2] = {2, 3, 4} etc.. You can build this with a simple BFS search. Then print all the nodes in depth[maxDepth], then those in depth[maxDepth - 1] etc.

The depth of a node i is equal to the depth of its father node + 1. The depth of the root node can be considered 1 or 0.

Answer 2

Use a combination of a stack and queue.

Do the 'normal' BFS using the queue (which I presume you know to do already), and keep pushing nodes on the stack as you encounter them.

Once done with the BFS, the stack will contain the reverse BFS order.