Why does this method cause an Infinite Recursive call?

Question

I'm struggling to understand why this class is not functioning. It was part of an assignment for a course on Data Structures(EDIT: The deadline for the assignment has passed, I just want to figure it out...). The node is part of an AVL tree built upon a BST and the way I chose to implement it is by creating methods within my Node class to find the Balance factor and height.

The class is structured as follows:

public class Node<T extends Comparable<? super T>> {

public T data;
public Node left;
public Node right;

public Node(T IN) {
    data = IN;
}

public Node(T IN, Node L, Node R) {
    this(IN);
    left = L;
    right = R;
}

@Override
public String toString() {
    return data.toString();
}

@Override
public Node clone() {
    return new Node(this.data) ;
}

public int getHeight() {
    return getHeight(this) ;
}

public int getBF() {

        //Calculate BF
        int balanceFactor = 0;
        if (right != null && left != null)
            balanceFactor = getHeight(right) - getHeight(left);
        else if (left != null) {
            balanceFactor = 0 - getHeight(left) ;
        }
        else if (right != null) {
            balanceFactor = getHeight(right) ;
        }
        else
            balanceFactor = 0 ;
        return balanceFactor ;
}

private int getHeight(Node p) {
    if (p.left == null && p.right == null ) {
        return 0 ;
    }
    else if (p.left != null && p.right != null) {
        return 1 + max(p.left.getHeight(), p.right.getHeight());
    }
    else if (p.left != null) {
        return 1 + p.left.getHeight() ;
    }
    else if (p.right != null) {
        return 1 + p.right.getHeight() ;
    }
    else {
        return 0;
    }
}

private int max(int x, int y) {
    if (x >= y) {
        return x;
    } else {
        return y;
    }
}

}

and the function calling the method is:

@Override
public boolean insert(T el) {
    boolean test = super.insert(el) ;
    if (test) {
        return checkBalance(root) ;
    }
    else
        return false ;
}

and the exception I recieve is a repetition of:

Exception in thread "main" java.lang.StackOverflowError
at Node.getHeight(Node.java:54)
at Node.getHeight(Node.java:33)
at Node.getHeight(Node.java:58)

Be sure left and right reference null in the default and in the "int Node(T IN)" constructors :) — Drewen, Apr 02 '13 at 10:28
Did you check if you have a circle in your tree? This may cause the error... — BobTheBuilder, Apr 02 '13 at 10:29
Well, in C++ is a "must" practice, but I just saw in Java it isn't :) — Drewen, Apr 02 '13 at 10:33
@Drewen It would be better practice, but Java sometimes makes me lazy... — FJ Lessing, Apr 02 '13 at 10:40

OldCurmudgeon · Accepted Answer · 2013-04-02T10:46:37.077

4

I would suggest that either your tree is deformed or really big. There seems to be no problems with the code.

If your tree is deformed in such a way that you have a Node inserted twice in the same tree then this code will break.

Added - You are eating a little more stack than you need - replacing p.left.getHeight() with getHeight(p.left) etc. would avoid one stack push per recursion. If your issue is merely big tree then this might scrape you through but this would only postpone the problem.

edited Apr 02 '13 at 10:46

answered Apr 02 '13 at 10:29

OldCurmudgeon

64,482
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If I test the getHeight() and getBF() methods with a simple binary tree it seems to work, I'll take a look at my AVL tree to see if it is broken, thanks. – FJ Lessing Apr 02 '13 at 10:36
I fixed it! The problem was in my AVL-tree's checkBalance() method, I had simply switched my rotations around and the problem existed because of the broken tree, so thank you to @OldCurmudgeon for making me look at my Data structure! – FJ Lessing Apr 02 '13 at 11:38

score 2 · Answer 2 · answered Apr 02 '13 at 10:28

2

From looking at both getHeight methods, it seems like you don't have a tree but a cyclic graph. You should start testing with a tree consisting of only the root and then add nodes until you observe the infinite recursion. You probably have an error in the function that rebalances the tree.

EDIT: And you should make the attributes (at least left and right) private.

answered Apr 02 '13 at 10:28

Axel

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"*From looking at both getHeight methods, it seems like you don't have a tree but a cyclic graph*" - how did you get to this conclusion? – SomeWittyUsername Apr 02 '13 at 10:31
Because if it's a noncyclic graph the methods as they are won't lead to infinite recursion. – Axel Apr 02 '13 at 11:44

Why does this method cause an Infinite Recursive call?

2 Answers2