Count number of nodes in binary tree in Java

Question

static int sum=0;
    public static int size(TreeNode root){
        if(root==null)
        return sum;
        sum++;
        sum=size(root.left);
        sum=size(root.right);
        return sum;
    }

We have to just complete the function "size" which counts the number of nodes in binary tree. I have written the above code. It is giving wrong answer for some testcases. Please explain what is wrong in above code.

Answer 1

Here:

sum=size(root.left);
sum=size(root.right);

You are computing two sums, to then throw away the first one!

You could go: return size(root.left)+size(root.right) + 1 instead.

There is also no point in using a static field sum here. If at all, that should be a local variable within that recursive method! Respectively: simply return 0 for null, otherwise use the return I provided here. There is no need to for that sum variable in the first place!

Answer 2

You have not set sum correctly when get data from child node

    sum += size(root.left);
    sum += size(root.right);

I would advice you not use a global static variable to get the value when you want to do it recursively

    static int sum=0;
    public static int size(TreeNode root){
        if(root==null)
        return 0;
        int cnt = 0;
        cnt++;
        cnt += size(root.left);
        cnt += size(root.right);
        return cnt;
    }

Answer 3

Try something like this:

public static int size(final TreeNode node)
{
    if (null == node ) return 0;
    return 1 + size( node.left ) + size( node.right );
}