Read Binary-Tree Values in Root-Left-Right Order
Company: Amazon
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: easy
Interview Round: Other
### Problem
Return a binary tree's values in preorder: visit the root, then its left subtree, then its right subtree. Omit absent children.
```cpp
vector<int> preorderTraversal(TreeNode* root);
```
The supplied `TreeNode` contains `val`, `left`, and `right`. Brackets in examples describe tree structure in level order.
### Examples
```text
Input: root = [7,3,11,1,5,null,13]
Output: [7,3,1,5,11,13]
```
```text
Input: root = []
Output: []
```
### Constraints
- There are between 0 and 100 nodes.
- Values lie between -100 and 100, inclusive.
### Optional follow-up
Can you perform the traversal iteratively instead of recursively?
Overview: Traverse binary-tree values in root-left-right order with an explicit stack that answers the iterative follow-up.
