Question

The question is how to create a binary tree, given its ancestor matrix. I found a cool solution at http://www.ritambhara.in/build-binary-tree-from-ancestor-matrics/. Problem is that it involves deleting rows and columns from the matrix. Now how do I do that? Can anybody suggest a pseudocode for this? Or, is there any better algo possible?

Answer 1

You don’t have to actually delete the rows and columns. You can either flag them as deleted in some additional array, or you can make them all zeros, which I think will be effectively the same (actually, you’ll still need to know that they are removed, so you don’t choose them again in step 4.c – so, flagging the node as deleted should be good enough).

Here are the modifications to the pseudocode from the page:

4.b.

used[temp] = true;
for (i = 0 to N) 
    Sum[i] -= matrix[i][temp]; (aka decrement sum if temp is a predecessor of i)
    matrix[i][temp] = 0;

4.c. Look for all rows for which Sum[i] == 0 and used[i] == false.