A decorator is just a callable that takes a function and returns a new one, often a wrapper, letting us extend behavior while keeping functions clean and following the Open–Closed Principle.

Several examples are presented to clarify the definition and application of type annotations in Python.

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

院角那株不知名的树，自漫长阴雨时节便浸在阴翳里。我总在窗前半掩着帘，看它瘦削的枝桠将天色裁成淡墨的写意，恍若故纸堆里褪了色的水墨小品。

The math is sound. The method is not. We consider the radial differential operator for a fixed angular mode \(n\), \[ L_n=\frac{d^2}{d r^2}+\frac{1}{r} \frac{d}{d r}-\frac{n^2}{r^2}, \quad r \in(0, \infty) \]