Java program to calculate how many red nodes are in a Red-Black Tree

Question

I'm building a program to determine how many red nodes are in a red black tree. I have already implemented a RED BLACK BST which builds the tree after reading a text input. I'm stuck on how to count the number of red nodes? I know the red nodes can only be left leaning and a red parent node can't have a red child. What would be the appropriate method on attacking this problem?

Please share the code you have so far (building the tree and counting nodes)! — alexbt, Oct 23 '16 at 00:09
You would pass the root node to your countRed method, than recursively call it again with left & right node (while calling isRed). You should give it a try! — alexbt, Oct 23 '16 at 20:51
@Alex I've updated my code! I'm currently getting a null pointer exception however — user3408536, Oct 23 '16 at 21:22
actually now I'm getting this error: non-static variable root cannot be referenced from a static context when i run it in my main method — user3408536, Oct 23 '16 at 21:34

alexbt · Accepted Answer · 2016-10-23T23:38:13.323

0

A recursive solution would look like this:

public static int countRed(Node node) {
    int nbRed = 0;
    if (node == null) {
        return 0;
    }
    nbRed += countRed(node.left);
    nbRed += countRed(node.right);

    if(node.isRed()){
        nbRed++;
    }
    return nbRed;
}

Tested

I tested your code with a simple loop:

RedBlackBST tree = new RedBlackBST();
tree.root = tree.put(null, 0, 0);

for (int i = 1; i < 10; i++) {
    tree.root = tree.put(tree.root, i, i);
}
int redTotal = tree.countRed(tree.root);

This returns a total of 3 reds. If I inspect it in debug, it's accurate. If you feel the value isn't valid, then you should inspect your tree in debug and identify what isn't valid in your structure.

edited Oct 23 '16 at 23:38

answered Oct 23 '16 at 21:38

alexbt

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thank you! However I still get the error: non-static variable root cannot be referenced from a static context when i run it in my main method... and when i change my root node to static then it says that non-static method node cannot be referenced from a static context when i run it in my main method? – user3408536 Oct 23 '16 at 21:47
Change 'tree.countRed(tree);' to 'tree.countRed(tree.root);' AND 'if (root.isRed(root))' to 'if (isRed(root))' – alexbt Oct 23 '16 at 22:16
Your code would benefit from a little refactoring. the method 'isRed' for instance => it should/could be a method in Node so you would call 'node.isRed()' instead of 'isRed(node)' – alexbt Oct 23 '16 at 22:18
thanks for all your help! It compiles and runs now however it doesn't count the nodes correctly so I guess i'll start over. – user3408536 Oct 23 '16 at 22:34
the code for the other methods was provided by the textbook so do you think the problem is in my count method then? – user3408536 Oct 23 '16 at 23:05
I tested creating a few node and calling countRed. I inspected the tree structure and the value was a match. If you expect another value, my guess is that the structure isn't built correctly. Anyhow, put a breakpoint and take some time to inspect your structure. You can at least confirm that the count is good, then the issue would be in rotateLeft/right (or the surrounding if-elses). – alexbt Oct 23 '16 at 23:40

Java program to calculate how many red nodes are in a Red-Black Tree

1 Answers1

Tested