BST中的节点计数

Question

I keep getting java.lang.StackOverflowError at this line:

count += countNodes(current.leftChild);

when I try to count the total number of the nodes in BST. Could somebody please tell me why I'm getting that error? Thanks in advance!

public int countNodes(Node node){
        if(root == null){
            System.out.println("The tree is empty!");
            return -1;
        }
        else{
            int count = 1;
            Node current = root;
            while(current.leftChild != null){
                count += countNodes(current.leftChild);
            }
            while(current.rightChild != null){
                count += countNodes(current.rightChild);
            }
            return count;
        }
    }

Answer 1

try this I think it should work

public int countNodes(Node root){
    if(root == null){
        System.out.println("The tree is empty!");
        return 0;
    } 
    else{
        Node current = root;
        int count = 0 
        if(current.leftChild != null){
            count +=  countNodes(current.leftChild)+1;
        }
        if(current.rightChild != null){
            count += countNodes(current.rightChild)+1;
        }
        return count;
    }
}

You go in recursion from the root to the leaves. Look at your code you go with while loop. The problem that in this recursion you go to the depth with the recursion and not with the while loop.

you take the result from the operation of the left tree and add it to the operation on the right. The stopping condition of the recursion is when the root dont have childs.

When

  current.rightChild == null && current.leftChild == null

Maybe I confused with the init of count. check it..

Answer 2

I see two problems with your original code.

The first is that you use while statements when looking at the left and right subtrees. Since you never change the value of current , these loops will run forever.

The second problem is that you take a parameter called node , but never use it. Instead, within the method body, you refer to another variable root . This is what is causing your StackOverflowError.

Changing the while s to if s should help with the infinite looping, and renaming some variables so that you use the value of parameter to the function should fix the stack overflow problem.

As a side note, you may want to return zero when the node is null instead of -1. That way, you can make recursive calls without having to check the null-ness of the children, and, more importantly, when the tree has zero nodes in it, the function says so. Right now, a tree with zero nodes looks like it has -1 nodes.

Answer 3

//^_^

public int getCount(){
    return getCount(root);
}

public int getCount(BSTNode<T> p){
    if(p == null){
        return 0;
    }
    else{
        int count = 1;
        BSTNode<T> current = p;
        while(current.left != null){
            count += getCount(current.left);
        }
        while(current.right != null){
            count += getCount(current.right);
        }
        return count;
    }
}