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

题目大意：展平二叉树

题目难度：Medium

/**
 * Created by gzdaijie on 16/6/9
 */
public class Solution {
    public void flatten(TreeNode root) {
        if (root == null) return;

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

        if (root.left != null) {
            TreeNode tmp = root.right;
            TreeNode tail = root.left;
            while (tail.right != null) tail = tail.right;
            root.right = root.left;
            tail.right = tmp;
            root.left = null;
        }
    }
}