将参考对象键设置为null不能按预期工作

Question

I need to remove the smallest value of binary search tree but I can't find a clean way to do it, currently I have kind of clean code, though it doesn't work as I expect it to work, I've got this code(MTE got MTE left, MTE right and int val keys):

MTE tempElement = root;

if(root == null) return;
else if((root.left == null) && (root.right == null))
{
    root = null;
    return;
}
else if(root.left != null)
{
    while(tempElement.left != null) tempElement = tempElement.left;

    if(tempElement.right == null)  tempElement = null;
    else tempElement = tempElement.right;
}
else if(root.right != null)
{
    if(tempElement.left == null) root = tempElement;
    else
    {
        while(tempElement.left != null) tempElement = tempElement.left;

        root.val = tempElement.val;
        if(tempElement.right == null) tempElement = null;
        else tempElement = tempElement.right;
    }
}

The problem I got with this code is when I get to this line of code - if(tempElement.right == null) tempElement = null; Which is 13th line on the code snippet that I provided. When I debugged it it changes tempElement to null, but my main root element doesn't change any of its nodes - the way I expected it to work, any workaround?

Answer 1

You should hold pointer to the parent node and then change the parent node's left or right pointer, for example

if(root.left != null)
{
    MTE prev = tempElement;
    while(tempElement.left != null) {
         prev = tempElement;
         tempElement = tempElement.left;
    }

    if(tempElement.right == null)  prev.left = null;
    else prev.left = tempElement.right;
}

Need to do the same for right != null as well.

Answer 2

Your left and right fields are reference variables. They can hold references to objects.

Your variable tempElement is a local variable. Initially it holds a copy of a reference to an object. It does not hold a reference to the field from which its value was copied. When you set that local variable to null, that's all that happens.

If you want to set a field in an object to null, you have to assign to the field -- not assign to a local variable that happens to hold a copy of the reference to the same object. For example, this will set a field to null:

tempElement.left = null;