Recursion

来自 Problem Solving with Algorithm and Data Structures Using Python，Release 3.0, Chapter 4, Page 117 学习笔记。

Definition¶

Recursion is a method of solving problems that involves breaking a problem down into smaller and smaller subproblems until you get into a small enough problem that it can be solved trivially. Usually recursion involves a function calling itself.

A recursive function is a function that calls itself.

The Three Laws of Recursion¶

A recursive algorithm must have a base case.
A recursive algorithm must change its state and move toward the base case.
A recursive algorithm must call, recursively