TODO

前后追击的双指针模板一直写不好

各类sort

注意mergesort对于链表不一样：需要先找mid，然后右边先，然后Mid.next=null再左边

N sum

dict sort in python

Set

& ||

time complexity

https://wiki.python.org/moin/TimeComplexity

Unit Test

design pattern

OOD

string:https://stackoverflow.com/questions/37133547/time-complexity-of-string-concatenation-in-python