The idea is to remove a row or column in each comparison until an element is found. Start searching from the top-right corner of the matrix.

Insert a new node after every node with same value
Make a copy
Separate the two lists

If we make sure that all even positioned (at index 0, 2, 4, ..) elements are greater than their adjacent odd elements, we don’t need to worry about odd positioned element.

I used the properties of preorder 
I used Recursion
I used HashMap for optimization

Create a graph
Traverse the two strings at a time, for forming an edge
Finally do Topological Sort