Trying to print top view of a tree using two if statements

Problem Statement

You are given a pointer to the root of a binary tree. Print the top view of the binary tree. You only have to complete the function.

My Code:

void top_view(Node root)
 {  
       Node r = root;

       if(r.left!=null){
          top_view(r.left);
          System.out.print(r.data + " ");
        }
       if(r.right!=null){
          System.out.print(r.data + " ");
          top_view(r.right);
        }
}

The two if statements are executed every time the function is called, but I need only one of them to execute. I tried switch but its giving constant expression error. I have already found a different solution for this problem.

So I only want to know if we can make only one if execute at a time i.e, is there a way to fix my code without changing the approach?

Problem link: https://www.hackerrank.com/challenges/tree-top-view

Answer

This problem can be very easily solved by using:

Stack: To print the root and the left subtree.

Queue: To print the right subtree.

Your function should be like this:

 void topview(Node root)
 {
     if(root==null)
      return;
     Stack<Integer> s=new Stack<Integer>();
     s.push(root.data);
     Node root2=root;
     while(root.left!=null)
     {
      s.push(root.left.data);
      root=root.left;
     }
     while(s.size()!=0)
      System.out.print(s.pop()+" ");

     Queue<Integer> q=new LinkedList<Integer>(); 
     q.add(root2.right.data);
     root2=root2.right;     
     while(root2.right!=null)
     {
      q.add(root2.right.data);
      root2=root2.right;
     }
     while(q.size()!=0)
      System.out.print(q.poll()+" ");
 }