Python Data Struct
Python Data Struct
Slicing
>>> s = 'bicycle'
>>> s[3:]
'ycle'
>>> s[:3]
'bic'
>>> s[::3]
'bye'
>>> s[::-1]
'elcycib'
If you want to reverse a string, the last example is a choice.
- assigning to slices
>>> l = list(range(10))
>>> l
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> l[2:5]
[2, 3, 4]
>>> l[2:5] = [20,30]
>>> l
[0, 1, 20, 30, 5, 6, 7, 8, 9]
what you can see is that [2,3,4] is replaced by [20,30]
List
list of list
>>> board = [['_'] * 3 for i in range(3)] >>> board [['_', '_', '_'], ['_', '_', '_'], ['_', '_', '_']] >>> board[1][2] = 'x' >>> board [['_', '_', '_'], ['_', '_', 'x'], ['_', '_', '_']]The first line is the right way to multiply it,rather than:
>>> wrong_board = [['_'] * 3] * 3 >>> wrong_board[1][2] = 0 >>> wrong_board [['_', '_', 0], ['_', '_', 0], ['_', '_', 0]]list.sort()&sorted(list)The
list.sort()method sorts a list in-place, that is, without making a copy.In contrast, the built-in function
sorted(list)creates a new list and returns it.找到列表中每一行的最大元素和每一列的最大元素
row = len(heights)
col = len(heights[0])
max_row = [0] * row
max_col = [0] * col
for i in range(row):
max_row[i] = max(heights[i])
for j in range(col):
for i in range(row):
max_col[j] = max(heights[i][j], max_col[j])
sort and sorted
sorted()
descending order (降序)
def max_n(lst, n=1, reverse=True): return sorted(lst, reverse=reverse)[:n]sort() + deepcopy()
ascending order (升序)
from copy import deepcopy def min_n(lst, n=1): numbers = deepcopy(lst) numbers.sort() return numbers[:n]
make list a stack or queue
The .append and .pop methods make a list usable as a stack or a queue (if you use .append and .pop(0), you get LIFO, Last in First out, behavior).
But inserting and removing from the left of a list (the 0-index end) is costly because the entire list must be shifted.
deques and queues
from collections import deque dq = deque(range(10), maxlen=10) # dq: deque([0, 1, 2, 3, 4, 5, 6, 7, 8, 9], maxlen=10) dq.rotate(3) # [7, 8, 9, 0, 1, 2, 3, 4, 5, 6] # this function rotates items from the right end # and when dp.rotate(-3) is from the left dq.appendleft(-1) # [-1, 1, 2, 3, 4, 5, 6, 7, 8, 9] dq.extend([11, 22, 33]) # [3, 4, 5, 6, 7, 8, 9, 11, 22, 33] # default is insert from rightWhat is different between
append()andextend()? here is an example:>>> dp # deque([10, 30, 20, 10, 3, 4, 5, 6, 7, 8], maxlen=10) >>> dp.appendleft([1, 2]) # deque([[1, 2], 10, 30, 20, 10, 3, 4, 5, 6, 7], maxlen=10) >>> dp.extendleft([1, 2]) # deque([2, 1, [1, 2], 10, 30, 20, 10, 3, 4, 5], maxlen=10)Note that
extendleft(iter)works by appending each successive item of the iter argument to the left of the deque, therefore the final position of the items is reversed.
Bisect
#bisect: [baɪ'sɛkt]
Bisection is the general activity of dividing a geometric figure into two equal parts
Set
Python 的集合是一个十分方便的对于元素可以操作的序列，除了去掉重复元素外，还可以进行稽核之间的运算。
student = {'Tom', 'Jim', 'Mary', 'Tom', 'Jack', 'Rose'}
print(student) # 输出集合，重复的元素被自动去掉
a = set('abracadabra')
b = set('alacazam')
print(a - b) # a 和 b 的差集
print(a | b) # a 和 b 的并集
print(a & b) # a 和 b 的交集
print(a ^ b) # a 和 b 中不同时存在的元素
set 的集合运算十分有用，看下面的代码：
class Solution:
def findWords(self, words):
"""
:type words: List[str]
:rtype: List[str]
"""
a = set('qwertyuiop')
b = set('asdfghjkl')
c = set('zxcvbnm')
ans = []
for word in words:
w = set(word.lower())
if (w & a == w) or (w & b == w) or (w & c == w):
ans.append(word)
return ans
上述代码实现了一个求解某序列是否在键盘的同一行的操作，通过求交集看是否结果等于自身就可以很方便地求解出结果。
set usage
- 使用 set 一般用于 判断一个值是否存在其中
- when to keep elements sorted and unique.
Example: 忽略常见单词，只对不在集合中的单词统计出现次数：
set<string> exclude = {"some", "words"};
//code
if(exclude.find(word) == exclude.end()) {
//code
}
对比如果使用 vector 实现：
vector<string> exclude = {"some", "words"};
//code
auto is_exclude = std::binary_search(exclude.cbegin(), exclude.cend(), word);
//bool binary_search()
auto reply = is_exclude ? "excluded" : "not excluded";