Grokking Meta Coding Interview

Solution: Diagonal Traverse

## Problem Statement
Given a 2D matrix of size `m x n`, return a 1D array containing elements of the matrix in the diagonal order.

### **Example 1:**
  - **Input:** `matrix = 
```
[[1, 2, 3, 4],
 [5, 6, 7, 8],
 [9, 10, 11, 12],
 [13, 14, 15, 16]]
```
  - **Expected Output:** `[1, 2, 5, 9, 6, 3, 4, 7, 10, 13, 14, 11, 8, 12, 15, 16]`
  - **Justification:** Traversal begins at 1, moves to 2, diagonally down to 5, and so on.

### **Example 2:**
  - **Input:** matrix = 
```
[[1,5],
 [3,6]]
```
  - **Expected Output:** `[1,5,3,6]`