> For the complete documentation index, see [llms.txt](https://rachel2011.gitbook.io/leetcode_cc150/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://rachel2011.gitbook.io/leetcode_cc150/data-structure/pythonji-qiao.md).

# python技巧

* 排序

用lst.sort() 而不是nlst = sorted(lst)

区别在于lst.sort()是 in-place sort，改变lst, sorted会创建新list，成本比较高。

* 用xrange

xrangge 和 range的区别是在于 range会产生list存在memory中，xrange更像是生成器，generate on demand

所以有的时候xrange会更快

* 三目运算符

python 的三目运算符是这么写的 x if y else z

考虑这种list of list: matrix = \[ \[1,2,3] , \[4,5,6] ]

row = len(matrix)

col = len(matrix\[0]) if row else 0

这样写通用的原因是， 当matrix = \[], row = 0, col =0

* list 填 0

lst = \[0 for i in range(3)] # lst = \[0,0,0]

lst = \[\[0 for i in range(3)] for j in range(2)] # lst = \[\[0, 0, 0], \[0, 0, 0]]

下面这种写法危险：

lst1 = \[ 0, 0, 0 ]

lst2 = \[lst1] \* 2 # lst2 = \[ \[0,0,0] , \[0,0,0] ]

lst2\[0]\[0] = 1 # lst2 = \[ \[1,0,0], \[1,0,0]]

因为lst1是object，这样写会踩坑

* D.get(key, default)

如果这个key 没有在dict里面，给它一个默认值：

D = {}

if 1 in D:

val = D\[1]

else :

val = 0

等同于这样写：

val = D.get(1, 0)

* reverse list

lst = \[1,2,3]

print lst\[::-1] #\[3,2,1]

lst 也有reverse函数

这也也适用于str, str可是没有reverse 函数的，str\[::-1] 可用 √

\=================进阶一下=====================

Python 也是有自己的数据结构的！！！！

* deque

  还在用list来做queue么？ deque，当求快queue的时候，你值得拥有
* Counter

  Counter做啥就顾名思义了
* yield

  用yield不用return （ 我也还在学习阶段

import collections就可以用了，参见

[collections — High-performance container datatypes](https://docs.python.org/2/library/collections.html)

\===============举个当时我就震惊了的例子===============

看到在stackoverflow上看到有人求这样一个数据结构：

[Anyone know this Python data structure?](http://stackoverflow.com/questions/4098179/anyone-know-this-python-data-structure)

* Close to

  **O(1) performance**

  for as many of the following four operations.
* Maintaining

  **sorted order while inserting**

  an object into the container.
* Ability to

  **peek at last value**

  (the largest value) contained in the object.
* Allowing for

  **pops on both sides**

  (getting the smallest or largest values).
* Capability of

  **getting the total size**

  or number of objects being stored.
* Being a

  **ready made solution**

  like the code in Python's standard library.

然后真的可以implement出来

1. import collections
2. import bisect
3. 1. class FastTable:
4. 1. def \_\_init\_\_(self):
5. self.\_\_deque = collections.deque()
6. 1. def \_\_len\_\_(self):
7. return len(self.\_\_deque)
8. 1. def head(self):
9. return self.\_\_deque.popleft()
10. 1. def tail(self):
11. return self.\_\_deque.pop()
12. 1. def peek(self):
13. return self.\_\_deque\[-1]
14. 1. def insert(self, obj):
15. index = bisect.bisect\_left(self.\_\_deque, obj)
16. self.\_\_deque.rotate(-index)
17. self.\_\_deque.appendleft(obj)
18. self.\_\_deque.rotate(index)

*复制代码*

对此我只想表示牛，并且我硬生生的用它来解过人家不是要这种思路的题目。

有想到的再补充，也欢迎任何技巧分享

**补充内容 (2016-10-28 00:40):**

Python有built-inheap,是min heap. heapq，如何来用它实现max heap呢，看到过一个有意思的方法是把key取负，比如把100变成-100，5变成-5。