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# 1028. Recover a Tree From Preorder Traversal

<https://leetcode.com/problems/recover-a-tree-from-preorder-traversal>

## Description

We run a preorder depth-first search (DFS) 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 `traversal` of this traversal, recover the tree and return *its* `root`.

**Example 1:**

![](https://assets.leetcode.com/uploads/2019/04/08/recover-a-tree-from-preorder-traversal.png)

```
**Input:** traversal = "1-2--3--4-5--6--7"
**Output:** [1,2,5,3,4,6,7]
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2019/04/11/screen-shot-2019-04-10-at-114101-pm.png)

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

**Example 3:**

![](https://assets.leetcode.com/uploads/2019/04/11/screen-shot-2019-04-10-at-114955-pm.png)

```
**Input:** traversal = "1-401--349---90--88"
**Output:** [1,401,null,349,88,90]
```

**Constraints:**

* The number of nodes in the original tree is in the range `[1, 1000]`.
* `1 <= Node.val <= 109`

## ac

```java
```