Tree : Top View of a Binary Tree

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. 
For example :

     3
   /   \
  5     2
 / \   / \
1   4 6   7
 \       /
  9     8
Top View : 1 -> 5 -> 3 -> 2 -> 7

Input Format

You are given a function,

void top_view(node * root)
{

}

Output Format

Print the values on a single line separated by space.

Sample Input

     3
   /   \
  5     2
 / \   / \
1   4 6   7
 \       /
  9     8

Sample Output

1 5 3 2 7

Explanation

     3
   /   \
  5     2
 / \   / \
1   4 6   7
 \       /
  9     8

From the top only nodes 1,5,3,2 and 7 will be visible.

/*
struct node
{
    int data;
    node* left;
    node* right;
};

*/
void left_view(node* root){
    if(root == NULL){
        return;
    }
    left_view(root->left);
    cout<<root->data<<" ";
}
void right_view(node* root){
    if(root == NULL){
        return;
    }
    cout<<root->data<<" ";
    right_view(root->right);
}
void top_view(node * root)
{
    if(root == NULL){
        return;
    }
    left_view(root->left);
    cout<<root->data<<" ";
    right_view(root->right);
}