68. Flatten Binary Tree to Linked List

 /**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void flatten(TreeNode root) {
        rec(root);
    }
   
    public TreeNode rec(TreeNode root) {
        if (root==null) return root;
       
        TreeNode leftsub = rec(root.left);
        TreeNode rightsub = rec(root.right);
       
        root.left = null;
        root.right = leftsub;
       
        TreeNode rightmost = leftsub;
       
        if(rightmost!=null) {
            while (rightmost.right!=null) {
                rightmost = rightmost.right;
            }
        }
       
        if (rightmost!=null) rightmost.right = rightsub;
        else root.right = rightsub;
       
        return root;
    }
}