1028. Recover a Tree From Preorder Traversal

We run a preorder depth first search on the root of a binary tree.

At each node in this traversal, we output D dashes (where D is the depth of this node), then we output the value of this node.  (If the depth of a node is D, the depth of its immediate child is D+1.  The depth of the root node is 0.)

If a node has only one child, that child is guaranteed to be the left child.

Given the output S of this traversal, recover the tree and return its root.

Example 1:

Input: "1-2--3--4-5--6--7"
Output: [1,2,5,3,4,6,7]

Input: "1-2--3---4-5--6---7"
Output: [1,2,5,3,null,6,null,4,null,7]