Concatenate Arrays

Problem Statement

Combine multiple NumPy arrays along different axes.

Background

• np.concatenate([arr1, arr2], axis=0): Stack vertically (default)
• np.concatenate([arr1, arr2], axis=1): Stack horizontally
• np.vstack(): Vertical stack
• np.hstack(): Horizontal stack

Your Task

Write a function concat_arrays(arr1, arr2) that returns a dictionary with:

• "vstack": Vertically stacked array as nested list
• "hstack": Horizontally stacked array as nested list
• "vstack_shape": Shape of vstack result as list
• "hstack_shape": Shape of hstack result as list

Output Format

Return a dictionary with exactly these four keys.