```
/**
* 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;
help(root);
}
TreeNode* help(TreeNode *root){
if(root == nullptr || (root->left == nullptr && root->right == nullptr))
return root;
TreeNode *temp = root->right,*temp1 = root->left, *t = root;
root->left = nullptr;
root->right = help(temp1);
while(root->right)
root = root->right;
root->right = help(temp);
return t;
}
};
```