Question

I have the following correct Java code to find the in-order kth element in a binary tree.

private static int count = 0;
public static <T> T findkthInOrder(Node<T> root, int k) {
    count=0;
    return findkthInOrder(root, k, 0);
}
public static <T> T findkthInOrder(Node<T> root, int k,int a) {
    if (root == null)
        return null;
    T rt = findkthInOrder(root.left, k, 0);
    if (rt != null)
        return rt;
    count++;
    if (count == k) {
        return root.data;
    }
    return findkthInOrder(root.right, k, 0);
}

But I really want to remove the use of count, possibly by making use of an additional method argument. I also want to keep it as recursion, and require the method findkthInOrder to return T type value.

Can anyone please help me with this? Thank you.

Answer 1

This is overkill but here goes…The only mutable number class I could find in Java was java.util.concurrent.atomic.AtomicInteger

public static <T> T findkthInOrder(Node<T> root, int k) {
    return findkthInOrder(root, k, new AtomicInteger(0));
}
public static <T> T findkthInOrder(Node<T> root, int k, AtomicInteger count) {
    if (root == null)
        return null;
    T rt = findkthInOrder(root.left, k, count);
    if (rt != null)
        return rt;
    if (count.incrementAndGet() == k) {
        return root.data;
    }
    return findkthInOrder(root.right, k, count);
}

I had made premature optimization, fixed now.

Here is alternate solution:

public static <T> T findkthInOrder(Node<T> root, int k) {
    return findkthInOrder(root, k, new int[]{0});
}
public static <T> T findkthInOrder(Node<T> root, int k, int[] count) {
    if (root == null)
        return null;
    T rt = findkthInOrder(root.left, k, count);
    if (rt != null)
        return rt;
    count[0]++;
    if (count[0] == k) {
        return root.data;
    }
    return findkthInOrder(root.right, k, count);
}