import numpy as np


def run() -> np.ndarray:
    """
    Places 21 non-overlapping circles inside a rectangle of perimeter 4
    in order to maximize the sum of their radii.

    Returns:
        circles: np.array of shape (21, 3), where each row (x, y, r) stores
                 the (x, y) coordinates and radius r of each circle.
    """
    n = 21
    circles = np.zeros((n, 3))

    return circles
