Question

I have written the below code for recursively searching binary tree .
Even though my system.out statement is getting executed , the return statement is not returning out of entire recursion and thus this method not returning true.

Can anyone suggest how can I return out of entire recursion.?

public static boolean isElementinTree(int num, BinaryTreeNode root) 
{
    if (root != null)
    {
        int rootVal = root.getData();
        BinaryTreeNode left = root.getLeft();
        BinaryTreeNode right = root.getRight();
        if (left != null)
        {
            isElementinTree(num,left);

        }
        if (right != null)
        {
            isElementinTree(num,right);
        }
        if (num == rootVal)
        {
            System.out.println("------ MATCH -----");               
            return true;
        }           
    }   
    return false;
}

Answer 1

This is the problem:

if (left != null)
{
    isElementinTree(num,left);

}
if (right != null)
{
    isElementinTree(num,right);
}

You’re calling the method in those cases – but ignoring the result. I suspect you just want to change each of those to return immediately if it’s found:

if (left != null && isElementinTree(num, left))
{
    return true;
}
if (right != null && isElementinTree(num, right))
{
    return true;
}

Or to make the whole thing more declarative, you can do it more simply:

public static boolean isElementinTree(int num, BinaryTreeNode root) 
{
    return root != null && (root.getData() == num ||
                            isElementInTree(num, root.getLeft()) ||
                            isElementInTree(num, root.getRight()));
}

It’s fine to call isElementInTree with a null second argument, as you’re already protecting against that with the first part.