Group Binary-Tree Values by Left-to-Right Level
Company: Google
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Other
### Problem
Return the values of a binary tree one level at a time, starting at the root. Within each level, list nodes from left to right. Return a list of value lists, with one inner list for each nonempty depth.
Preserve the supplied `TreeNode` interface with `val`, `left`, and `right` fields:
```cpp
vector<vector<int>> levelOrder(TreeNode* root);
```
The examples display input trees in level order with `null` for missing children; that notation is not an array-based replacement for the pointer argument.
### Examples
```text
Input: root = [10,4,15,null,6,12,18]
Output: [[10],[4,15],[6,12,18]]
```
```text
Input: root = [-5,null,2,null,7]
Output: [[-5],[2],[7]]
```
### Constraints
- The tree has between 0 and 2000 nodes.
- Node values lie between -1000 and 1000, inclusive.
- Missing children are not output values. An empty tree returns an empty collection.
Overview: Traverse a binary tree by depth with left-to-right values grouped into one list per level.