Vertical sum in a binary tree

Refer : http://www.geeksforgeeks.org/archives/18215

import java.util.Collections;
import java.util.Map;
import java.util.HashMap;
import java.util.List;
import java.util.ArrayList;
import java.util.Set;
import java.util.Iterator;

public class VerticalSumInTree {
 public static void main(String[] args) {
  Node root = getTree();
  printVertcalSumsInTree(root);
 }

 private static Node getTree() {
  Node node4 = new Node(4);
  Node node5 = new Node(5);
  Node node6 = new Node(6);
  Node node7 = new Node(7);
  Node node2 = new Node(2, node4, node5);
  Node node3 = new Node(3, node6, node7);
  Node root = new Node(1, node2, node3);
  return root;
 }

 private static void printVertcalSumsInTree(Node root) {

  if (root == null)
   return;

  // Used for storing nodes matches to same horizontal distance
  Map<Integer, List<Node>> map = new HashMap<Integer, List<Node>>();
  calculateVerticalSum(root, map, 0);

  printVerticalSums(map);
 }

 private static void printVerticalSums(Map<Integer, List<Node>> map) {
  if (map == null) {
   System.out.println("Tree is empty");
  } else {
   Set<Integer> keys = map.keySet();

   // Sort the keys
   // convert set to list for sorting.Because collections accepts list
   // for sorting
   List<Integer> keysList = new ArrayList<Integer>(keys);
   Collections.sort(keysList);
   // For each key, get the list of values
   for (Integer key : keysList) {
    List<Node> list = map.get(key);

    // Iterate through the list elements and calculate the sum
    int sum = 0;
    Iterator<Node> it = list.iterator();
    while (it.hasNext()) {
     Node n = it.next();
     sum = sum + n.value;
    }
    System.out.println(sum);
   }
  }
 }

 private static void calculateVerticalSum(Node root,
   Map<Integer, List<Node>> map, int horizontalDistance) {

  if (root == null)
   return;

  // Add this value to the map
  List<Node> list = map.get(horizontalDistance);
  if (list == null) {
   list = new ArrayList<Node>();
  }

  list.add(root);
  map.put(horizontalDistance, list);

  // iterate for left and right trees
  calculateVerticalSum(root.left, map, horizontalDistance - 1);
  calculateVerticalSum(root.right, map, horizontalDistance + 1);
 }

 public static class Node {

  public int value;

  // member variables will be initialized to null by default.
  public Node left;
  public Node right;

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

  public Node(int value, Node left, Node right) {
   this.value = value;
   this.left = left;
   this.right = right;
  }

 }
}