How to check if a Binary Search Tree is Perfectly Balanced?

Question

I have this homework problem and I have completed all methods except this one, isPerfectlyBalanced().

All my tests pass except one that should return false but instead returns true. I have attached my current code and the test that is failing. Any description on how to go about this or even let me know where my code is wrong is appreciated!

private boolean isPerfectlyBalanced(Node node) {

    if (node == null) {
        return true;
    }

    if(size(node.left) == size(node.right)) {
        return true;
    }
    isPerfectlyBalanced(node.left);
    isPerfectlyBalanced(node.right);
    return false;

}


public boolean isPerfectlyBalancedS() {
    // TODO
    if (root == null) {
        return true;
    }
    return isPerfectlyBalanced(root);

}

Here is my test that is failing:

assertFalse(set.isPerfectlyBalancedS());

Thank you!

My size method:

private int size(Node node){
    if (node == null){
        return 0;
    } else {
        return (size(node.left) + 1 + size(node.right));
    }
}
public int size() {
    // TODO
    return size(root);
}

Note you ignore the return values of the recursive `isPerfectlyBalanced()` invocations. — Gyro Gearless, Apr 29 '19 at 14:01
https://stackoverflow.com/questions/20507665/how-to-determine-a-balanced-or-perfectly-balanced-binary-search-tree-just-from — Joop Eggen, Apr 29 '19 at 14:08

score 5 · Accepted Answer · answered Apr 29 '19 at 14:01

5

On the last line of the first method, you probably want to do this:

return (isPerfectlyBalanced(node.left) && isPerfectlyBalanced(node.right));

instead of

isPerfectlyBalanced(node.left);
isPerfectlyBalanced(node.right);
return false;

In your code, you dismiss the result of the isPerfectlyBalanced on the subtrees and always return false.

answered Apr 29 '19 at 14:01

NeplatnyUdaj

6,052
6
43
76

Perfect! To fix it, I had to make the change in the line if(size(node.left) == size(node.right)) { ....., after I just had a return false call at the end of everything. I see where I was wrong by just jumping over everything. Maybe I need to practice on my recursion a bit more. – QWERTY Apr 29 '19 at 15:23

How to check if a Binary Search Tree is Perfectly Balanced?

1 Answers1