Recursion

Self-reference

都是返回函数导致的 😋

然后函数再次被call，就形成了自我递归

Recursive Functions

有一个细节，递归调用的frame的parent frame都是global而不是nested 🤔

Mutual Recursion

互相调用

Recursion to Iteration

key: figure out what state must be maintained during the while loop

Iteration to Recursion

Iteration is a special case of recursion 😮