Traverse Binary-Tree Values in Left-Root-Right Order
Company: Google
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: easy
Interview Round: Other
### Problem
Return the node values of a binary tree in inorder: traverse the left subtree, then the current node, then the right subtree. The input is an arbitrary binary tree, not necessarily a binary search tree.
Use the supplied `TreeNode`, with `val`, `left`, and `right` fields:
```cpp
vector<int> inorderTraversal(TreeNode* root);
```
Tree examples use level-order notation, with `null` for a missing child. The actual input remains a node pointer.
### Examples
```text
Input: root = [4,2,7,null,3,6]
Output: [2,3,4,6,7]
```
```text
Input: root = [9,null,-1]
Output: [9,-1]
```
### Constraints
- The tree has between 0 and 100 nodes.
- Node values lie between -100 and 100, inclusive.
### Source follow-up
In addition to the recursive method, perform the traversal iteratively.
Overview: Return an arbitrary binary tree’s left-root-right value traversal, with recursive and iterative methods.