Assume we have a tree and we want to serialize it. Example: ``` Tree 1 / \ 3 2 / \ / \ 5 N N N / \ N N ``` can result as: ``` "1,3,5,N,N,N,2,N,N", // Pre Order serialization; "N,5,N,3,N,1,N,2,N", // In Order serialization; "N,N,5,N,3,N,N,2,1", // Post Order serialization ``` I would like to know, which one of these three serializations is unique for all trees and why ?