/**

  * Definition for a binary tree node.

  * struct TreeNode {

  *     int val;

  *     TreeNode *left;

  *     TreeNode *right;

  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

  * };

  */

 class Solution {

 public:

     void flatten(TreeNode* root) {

         if (root == nullptr) {

             return;

         }

         while (root != nullptr) {

             if (root -> left != nullptr) {

                 TreeNode* temp = root -> left;

                 while (temp -> right != nullptr) {

                     temp = temp -> right;

                 }

                 temp -> right = root -> right;

                 root -> right = root -> left;

                 root -> left = nullptr;

             }

             root = root -> right;

         }

     }

 };