Simple solution like creating ll from tree

```
TreeNode* Solution::flatten(TreeNode* A)
{
if(A==NULL)
return NULL;
TreeNode* temp1=flatten(A->left); //left list
TreeNode* temp2=flatten(A->right); //right list
A->right=temp1; //connect root to l1
A->left=NULL; //make left null
if(temp1!=NULL) //connect l1 to l2
{
while(temp1->right!=NULL)
temp1=temp1->right;
temp1->right=temp2;
}
else
A->right=temp2; //connect root to l2 if l1 is null
return A;
}
```