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```/**
* 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:
/* we take the root and solve its LEFT and RIGHT parts , then we wtich it back properly first by setting root->left= NULL and then we set root->right=LEFT;
then we go to the end of right of updated root and attach orininal RIGHT there */

void flatten(TreeNode* root) { //recursion
if(root==NULL) return;
TreeNode* left =root->left;
TreeNode* right=root->right;

root->left=NULL;

flatten(left);
flatten(right);
root->right=left;
TreeNode* tmp=root;
while(tmp->right!=NULL) tmp=tmp->right;
tmp->right=right;
}
};

/*
class Solution {
public:
void flatten(TreeNode* root) { //pre order travrsal
TreeNode* curr=root;
while(curr!=NULL){
if(curr->left!=NULL){
TreeNode* pred=curr->left;
while(pred->right!=NULL){
pred =pred->right;
}
pred->right= curr->right;
curr->right=curr->left;
curr->left=NULL;
}
curr=curr->right;
}
}
}; */```