Medium · Trees

Convert binary tree to doubly linked list

Given the root of a binary tree, return the head of a new non-circular doubly linked list that holds the tree's values in in-order sequence (left subtree, node, right subtree). Each list node's Prev and Next point to its neighbours, both ends are nil, and an empty tree returns null.

{
  "tree": [4, 2, 5, 1, 3]
}

{
  "tree": [5, 3, 8, 2, 4, null, 9]
}