Given a Binary Tree and a number k, remove all nodes that lie only on root to leaf path(s) of length smaller than k. If a node X lies on multiple root-to-leaf paths and if any of the paths has path length >= k, then X is not deleted from Binary Tree. In other words a node is deleted if all paths going through it have lengths smaller than k.

Consider the following example Binary Tree 
 

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

Input: Root of above Binary Tree
      k = 4

Output: The tree should be changed to following  

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

There are 3 paths 
i) 1->2->4->7      path length = 4
ii) 1->2->5        path length = 3
iii) 1->3->6->8    path length = 4 

There is only one path " 1->2->5 " of length smaller than 4.  
The node 5 is the only node that lies only on this path, so 
node 5 is removed.

Nodes 2 and 1 are not removed as they are parts of other paths
of length 4 as well.

If k is 5 or greater than 5, then whole tree is deleted.

If k is 3 or less than 3, then nothing is deleted.

Remove nodes on root to leaf paths of length less than K: https://www.geeksforgeeks.org/remove-nodes-root-leaf-paths-length-k/