二叉树展开为链表

题目来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/flatten-binary-tree-to-linked-list

给定一个二叉树，原地将它展开为链表。

例如，给定二叉树

    1
   / \
  2   5
 / \   \
3   4   6

将其展开为：

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

递归解法：

/**
 * 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 || (root.left == null && root.right == null))
            return;
        flatten(root.left);
        flatten(root.right);
        //到这一步，root节点下面最多就只有俩节点
        if (root.left != null){
            TreeNode tmpRight = root.right;
            root.right = root.left;
            root.left = null;
            TreeNode r = root.right;
            while(r.right != null){
                r = r.right;
            }
            r.right = tmpRight;
        }
    }
}