Superimpose binary tree

Problem

Given two binary trees, find the resultant binary tree if we put one binary tree on top of another. For example,

          Below

             9

         /       \

      3          5

    /            /  \

  2            4    6

       Above

           7

       /      \

    3          8

              /

           1

              \

               5

         Result

            7

         /     \

       3        8

     /         /    \

   2        1        6

               \

                 5

Just look how the 2 and 6 are visible from the below tree and rest are covered by above tree.

Solution

We will do a preorder traversal along the two trees simultaneously, When above is null we will return the below node. when below is null we don't need to proceed further as the above tree will remain same for that subtree. So we return above when below is null. When above and below both are non null we will call the superimpose recursively to the left and right subtree.

Code

public class SuperImposeBinaryTree
{
 public static void main(String[] args)
 {
  Node a = new Node(1);
  Node b = new Node(2);
  Node c = new Node(3);
  Node d = new Node(4);
  Node e = new Node(5);
  Node f = new Node(6);
  a.left = b;
  a.right = c;
  b.left = d;
  c.left = e;
  c.right = f;

  Node u = new Node(4);
  Node v = new Node(7);
  Node w = new Node(2);
  Node x = new Node(8);
  Node y = new Node(1);
  Node z = new Node(5);

  u.left = v;
  u.right = w;
  w.left = x;
  w.right = y;
  y.right = z;
  printNice(a);
  System.out.println();
  printNice(u);
  System.out.println();
  Node result = superImpose(a, u);
  printNice(result);
 }

 private static Node superImpose(Node below, Node above)
 {
  if (above == null)
   return below;
  if (below == null)
   return above;
  above.left = superImpose(below.left, above.left);
  above.right = superImpose(below.right, above.right);
  return above;
 }

 public static void printNice(Node root)
 {
  if (root == null)
   return;
  else
  {
   System.out.print(root.value);
   if (root.left != null)
   {
    System.out.print("L->[");
    printNice(root.left);
    System.out.print("]");
   }
   if (root.right != null)
   {
    System.out.print("R->[");
    printNice(root.right);
    System.out.print("]");
   }
  }
 }

 static class Node
 {
  Node left;
  Node right;
  int value;

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

  public static void printNice(Node root)
  {
   if (root == null)
    return;
   else
   {
    System.out.print(root.value);
    if (root.left != null)
    {
     System.out.print("L->[");
     printNice(root.left);
     System.out.print("]");
    }
    if (root.right != null)
    {
     System.out.print("R->[");
     printNice(root.right);
     System.out.print("]");
    }
   }
  }
 }
}