Flatten Binary Tree to Linked List

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.

/**
 * 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) {
        if (root == null) {
            return;
        }

        flatten(root.left);
        flatten(root.right);

        TreeNode leftTmp = getLastNode(root.left);
        if (leftTmp != null) {
            leftTmp.right = root.right;
            root.right = root.left;
            root.left = null;
        }
    }

    public TreeNode getLastNode(TreeNode root) {
        TreeNode result = root;
        if (result != null) {
            while (result.right != null) {
                result = result.right;
            }
        }
        return result;
    }
}