114. 二叉树展开为链表

例如，给定二叉树

    1
   / \
  2   5
 / \   \
3   4   6
将其展开为：

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public void flatten(TreeNode root) {
        if (root == null) {
            return;
        }

        TreeNode left = root.left;
        TreeNode right = root.right;

        root.left = null;

        flatten(left);
        flatten(right);

        root.right = left;
        TreeNode cur = root;
        while (cur.right != null) {
            cur = cur.right;
        }
        cur.right = right;
    }
}