Red Black Tree <Black Height> (Redraft)
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05-09-2019 - |
Question
/** The following function checks the red black tree black height
* @param n the root node is inputed then a traversal is done to calculate the black-height
* @return Return an error message / mesages informing the user whether or not the black height was maintained
* @author Ferron Smith
*/
public static void getCount (SkaRedBlackTreeNode skaRedBlackTreeNode) {
VizRedBlackTreeNode n = skaRedBlackTreeNode.getVizRep();
if (validRoot(n))
{
static int lcount = leftCount(n);
static int rcount = rightCount(n);
if (rcount == lcount) {
n.displayMsg("Black height maintained");
}
else
// n.displayWarning("rcount " + rcount + " lcount " + lcount);
n.displayError("Red Black Tree is unbalanced");
}
}
/** The following function counts all the black node of the left side of the tree
* @param n the left child is inputed and a traversal is done to count all the black nodes
* */
public static int leftCount (VizRedBlackTreeNode n)
{
if (n == null)
return 0;
else if (n.getrbtColr() == Color.black)
return 1 + leftCount(n.getLeft());
else
leftCount(n.getLeft());
}
/** The following function counts all the black node of the right side of the tree
* @param n the right child is inputed and a traversal is done to count all the black nodes
* */
public static int rightCount (VizRedBlackTreeNode n)
{
if (n == null)
return 0;
else if (n.getrbtColr() == Color.black) {
return 1 + rightCount (n.getRight());
else
rightCount(n.getRight());
}
}
This is redraft, do you think this one will work, i've tested it on certain conditions and as not failed me yet
Solution
As far as I can tell, you're checking black height only on the leftmost and rightmost paths down the tree. The definition of a red-black tree requires that black height be the same on all paths. For example, this invalid tree is not flagged by your program:
B
/ \
/ \
/ \
B B
/ \ / \
B R R B
Also, it doesn't check for cycles or if the keys are in order.
OTHER TIPS
So I realize that you're working in java here, but here's some pseudocode that may help:
unsigned int blackHeight()
height, heightLeft, heightRight = 0
if black
height++
if left
heightLeft = left->blackHeight()
else
heightLeft = 1
if right
heightRight = right->blackHeight()
else
heightRight = 1
if heightLeft != heightRight
//YOU HAVE A PROBLEM!
height += heightLeft
return height
I'm only just starting to experiment with red black trees myself, but I believe this algorithm should give you the black height on any node you call it from.
Edit: I guess I should qualify, this would be code found within a node not within the tree. In c++ it would be called with a someNode->blackHeight().
//enter code here
private static void blackHeight(rbnode root) {
if (root == null)
return;
if (root.color == "black") {
root.black_count = root.parent.black_count+1;
} else {
root.black_count = root.parent.black_count;
}
if ((root.left == null) && (root.right == null)) {
bh = root.black_count;
}
blackHeight(root.left);
blackHeight(root.right);
}