Given a binary tree, flatten it to a linked list in-place.

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

Hints:

If you notice carefully in the flattened tree, each node’s right child points to the next node of a pre-order traversal.

public class Solution {
    public void flatten(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(root==null){
            return;
        }
        if(root.left==null){
            flatten(root.right);
            return;
        }
        
        TreeNode left = root.left;
        TreeNode right = root.right;
        root.left = null;
        if(right!=null){
            //find the right most element in root's left subtree
            //add root's right subtree to the element as a right child
            TreeNode runner = left;
            while(runner.right!=null){
                runner = runner.right;
            }
            runner.right = right;
        }
        root.right=left;
        flatten(left);
        
    }

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