Given below is a binary tree. The task is to print the top view of binary tree. Top view of a binary tree is the set of nodes visible when the tree is viewed from the top. For the given below tree

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

Top view will be: 4 2 1 3 7
Note: Return nodes from leftmost node to rightmost node.

Example 1:

Input:
      1
   /    \
  2      3
Output: 2 1 3

Example 2:

Input:
       10
    /      \
  20        30
 /   \    /    \
40   60  90    100
Output: 40 20 10 30 100

Your Task:
Since this is a function problem. You don't have to take input. Just complete the function topView() that takes root node as parameter and returns a list of nodes visible from the top view from left to right.

Expected Time Complexity: O(N)
Expected Auxiliary Space: O(N).

Constraints:
1 ≤ N ≤ 10⁵
1 ≤ Node Data ≤ 10⁵
 

Note:The Input/Ouput format and Example given are used for system's internal purpose, and should be used by a user for Expected Output only. As it is a function problem, hence a user should not read any input from stdin/console. The task is to complete the function specified, and not to write the full code.