```
/**
* 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)
{
for (TreeNode *cur = root, *tmp = nullptr; cur; ) {
if (cur->left) {
for (tmp = cur->left;
tmp->right && tmp->right != cur;
tmp = tmp->right) ;
if (tmp->right) {
/* everything else is standard Morris except for this if section. */
tmp->right = cur->right;
tmp = cur->right;
cur->right = cur->left;
cur->left = nullptr;
cur = tmp;
} else {
tmp->right = cur;
cur = cur->left;
}
} else {
cur = cur->right;
}
}
}
};
```