You don't need parent pointers. For converting binary tree into a linked list in O(n) time and O(1) space. You have two pointers, the root and the tail (right

You don't need parent pointers. For converting binary tree into a linked list in O(n) time and O(1) space. You have two pointers, the root and the tail (right child node). If root has a left subtree, set tail->right = left subtree. Update tail again so it is the last right child node. root = root->right Repeat until root is NULL