Given a BST and a node (say target), find K nearest neighbors. http://www.geeksforgeeks.org/print-nodes-distance-k-given-node-binary-tree/

bst.java

package com.mycompany.tree;

import java.util.*;
import java.util.List;

public class BinarySearchTree {

    private Node root;

    public List<Node> printKNeighbours(int value, int k) {

        List<Node> path = pathToRoot(value);
        if (path.size() == 0){
            return Collections.EMPTY_LIST;
        }

        for (int i = 0; i < path.size(); i++) {
            Node current = path.get(i);
            current.level = i;
        }

        return fetchkNeighbours(k);
    }

    private List<Node> fetchkNeighbours(int k) {

        List<Node> kList = new ArrayList<Node>();
        LinkedList<Node> list = new LinkedList<Node>();

        list.add(root);
        if (root.level == k) {
            kList.add(root);
        }

        while (!list.isEmpty()) {

            Node node = list.poll();

            if (node.left != null) {
                node.left.level = node.level + 1;
                list.addFirst(node.left);
                if (node.left.level == k){
                    kList.add(node.left);
                }
            }

            if (node.right != null) {
                node.right.level = node.level + 1;
                list.addFirst(node.right);
                if (node.right.level == k){
                    kList.add(node.right);
                }
            }
        }

        return kList;
    }


    private List<Node> pathToRoot(int value) {

        List<Node> normal = findPath(value);
        Collections.reverse(normal);

        return normal;
    }
    /*
        Returns a path to the node containing value. Otherwise returns empty list
     */
    private List<Node> findPath(int value) {

        if (root == null) {
            return Collections.EMPTY_LIST;
        }

        List<Node> path = new ArrayList<Node>();
        Node current = root;

        while (current != null) {

            path.add(current);
            if (current.value == value) {
                break;
            }
            if (value < current.value) {
                current = current.left;
            }else {
               current = current.right;
            }
        }

        return current != null ? path : Collections.EMPTY_LIST;
    }
}

Node.java

package com.mycompany.tree;


public class Node {
    public int value;
    public Node left;
    public Node right;
    public int level;

    public Node(int value) {
        this.value = value;
        this.level = -1;
    }
}