给定一个二叉树，原地将它展开为一个单链表。

    例如，给定二叉树

    1. 1
    2.    / \
    3.   2   5
    4.  / \   \
    5. 3   4   6

    将其展开为：

    1
2
3
4
5
6
7
8
9
10
11
1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

    1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
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/*
 * @lc app=leetcode.cn id=114 lang=cpp
 *
 * [114] 二叉树展开为链表
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode* root) {
        if(root == nullptr)  return ;

        flatten(root->left);
        flatten(root->right);

        TreeNode* left, *right;
        left = root->left;
        right = root->right;
        root->right  = left;
        root->left = nullptr;
        TreeNode* p = root;
        while(p->right){
            p = p->right;
        }
        p->right = right;

    }
};
// @lc code=end