What is a sustabable nodes?

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What is a sustabable nodes?

I have the nodes g (green) and b (black.
Here's an assigment:

This exercise is about rooted binary trees. Each node x has fields x.parent, x.left and x.right denoting the parent, left
child, and right child of x. The root of the tree is denoted by root and root.parent = null. Furthermore, each node x also
has a field x.color, called the color of x, that can be either green or black. Throughout the exercise, we let n denote the
size of the input tree.

2.1 (2 %) A node x is sustainable if the color of x is green and the color of all descendants of x is also green. Consider
the tree below. Nodes with a g are green and nodes with a b are black. Mark all sustainable nodes in the tree. Do so
directly on the vertices in the tree.

What I have tried:

I've never heard of sustainable nodes, how should that work?

Posted 18-May-20 22:11pm

Member 14836053

Updated 18-May-20 23:14pm

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OriginalGriff 19-May-20 4:14am

This is not a good question - we cannot work out from that little what you are trying to do.
Remember that we can't see your screen, access your HDD, or read your mind - we only get exactly what you type to work with.
Use the "Improve question" widget to edit your question and provide better information.

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OriginalGriff · Answer 1 · 2020-05-18T23:15:00

Thank you for the update - it helps to give us information, even if it's only your actual homework question!

So read the question carefully - it specifies exactly what a "sustainable node" is:

Quote:
A node x is sustainable if the color of x is green and the color of all descendants of x is also green.

I.e. is a node is Green and all it's "child nodes" are also all Green then the node is sustainable. Any Black nodes in a nodes descendants make it non-sustainable.

So create your tree, fill it with the appropriate data, and search the tree to identify sustainable nodes.

Hint: I'd start by setting all nodes as non-sustainable and traverse the tree from the bottom up. Hitting a Black node prevents any nodes above it from being sustainable, so there is no point in traversing them.