text: /*
   * @lc app=leetcode id=114 lang=cpp
   *
   * [114] Flatten Binary Tree to Linked List
   *
   * https://leetcode.com/problems/flatten-binary-tree-to-linked-list/description/
   *
   * algorithms
   * Medium (53.58%)
   * Total Accepted:    481.6K
   * Total Submissions: 886.1K
   * Testcase Example:  '[1,2,5,3,4,null,6]'
   *
   * Given the root of a binary tree, flatten the tree into a "linked
   * list":
   * 
   * 
   * The "linked list" should use the same TreeNode class where the right child
   * pointer points to the next node in the list and the left child pointer is
   * always null.
   * The "linked list" should be in the same order as a pre-order traversal of
   * the binary tree.
   * 
   * 
   * 
   * Example 1:
   * 
   * 
   * Input: root = [1,2,5,3,4,null,6]
   * Output: [1,null,2,null,3,null,4,null,5,null,6]
   * 
   * 
   * Example 2:
   * 
   * 
   * Input: root = []
   * Output: []
   * 
   * 
   * Example 3:
   * 
   * 
   * Input: root = [0]
   * Output: [0]
   * 
   * 
   * 
   * Constraints:
   * 
   * 
   * The number of nodes in the tree is in the range [0, 2000].
   * -100 <= Node.val <= 100
   * 
   * 
   * 
   * Follow up: Can you flatten the tree in-place (with O(1) extra space)?
   */
  /**
   * 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) {}
   * };
   */
  
  #include "common.hpp"
  
  class Solution {
  public:
      void flatten(TreeNode* root) {
          if (root == nullptr) return;
          vector<TreeNode*> preorder_lst;
          this->preorder(preorder_lst, root);
          for (int i = 0; i < preorder_lst.size() - 1; i++) {
              auto node = preorder_lst.at(i);
              node->left = nullptr;
              node->right = preorder_lst.at(i + 1);
          }
          auto last_node = preorder_lst.at(preorder_lst.size() - 1);
          last_node->left = nullptr;
          last_node->right = nullptr;
      }
  
  private:
      void preorder(vector<TreeNode*> &preorder_lst, TreeNode* root) {
          if (root == nullptr) return;
          preorder_lst.push_back(root);
          this->preorder(preorder_lst, root->left);
          this->preorder(preorder_lst, root->right);
      }
  };
  