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1.1 Python Compact Coding
Name Comment Return if.. else return val if i>0 else 0Multiple assignment l, r = 2, 3Assign with check of none a = b if b else 1Assignments l[1]=l[0]=0Swap values left, right = right, leftList Comprehensions [x*x for x in range(1, 1001)]List Comprehensions l = [2, 3, 5]; [2*x for x in l if x>2]Use zip for a, b in zip(nums, nums[3:])Build a list dp = [1] + [0]*3Change interger to string in binary bin(num), f'{num:b}', "{0:b}".format(num)Sum a subarray sum(nums[0:k])Sort list in descending order sorted(nums, reverse=True)Dictionary with defaults m = collections.defaultdict(lambda: 1)Loop with single statement while p.left: p = p.leftPrint multiple values print(x, y)Get both index and item for i, ch in enumerate(["a", "b", "c"]): print(i, ch)Mod negative (-2)%5Compare values if 0<=i<n and 0<=j<m and grid[i][j]if … return if k == 0: return False if… continue if index == icol: continue List comprehensive areas = [dfs(i, j) for i in range(m) for j in range(n) if grid[i][j]] Python assertion assert [1,2]==[1,2]
1.2 Python Advanced: Concepts & Internals
Name Comment Create a fixed size array [None]*5Create a fixed size matrix/2D array [[sys.maxsize for j in range(2)] for i in range(3)]Flatten 2D array into 1D array [a for r in matrix for a in r]Iterate over a list for v in l:Iterate over a list with index+val for i, v in enumerate(l):zip two lists as one l = sorted(zip(nums, range(len(nums)))) Convert int array to a string ' '.join([str(v) for v in [1, 2,3,4]])Extact columns from multi-dimensional array [row[1] for row in l]Sort in descending l=sorted([8, 2, 5], reverse=True) Sort list by a lambda key l=sorted([(‘ebb’,12),(‘abc’,14)], key=lambda x: x[1]) Sort list by a function sorted(logs, key=getKey), LeetCode: Reorder Data in Log Files In-place sort l.sort()Find the index of one item [1,2,5,3].index(2)Return all but last list[:-1]The second last item list[-2] or list[~1]Generate a-z map(chr, range(ord('a'), ord('z')+1))Convert from ascii to character chr(ord('a'))Reverse a list ["ab", "cd", "ef"][::-1]map map(lambda x: str(x), [1, 2, 3])Copy a range to another range nums1[:k+1] = nums2[:j+1] append an element array.append(var)insert elements to head array.insert(0,var)delete element by index del a[1]list as stack item = l.pop()map/reduce functools.reduce((lambda x, y: "%s %s" % (x, y)), l)replace ith to jth list[i:j] = otherlistcombine two list list1 + list2get sum sum(list)unique list set(["Blah", "foo", "foo", 1, 1, 2, 3])Insert to sorted list bisect.insort(l, 3)Reverse a list l[::-1]Print zip array print(list(zip(l1, l2)))Reference Link: Lists and Tuples in Python
Name Comment Reverse string ‘hello world’[::-1] Array to string ’ ‘.join([‘a’, ‘b’]) Integer array to string ’ ‘.join([str(v) for v in [1, 2, 3]]) Split string to array “hello, python”.split(“,”) String to array list('abc')Format to 2 digits print "%02d" % (13)Capitalize string ‘hello world’.capitalize() Upper/lower string ‘aBc’.upper(), ‘aBc’.lower() Check if string represent integer ‘123’.isdigit() Check if string alphabetic ‘aBc’.isalpha() Check if string alphanumeric ‘a1b’.isalnum() Count substring ‘2-5g-3-J’.count(‘-‘) Remove tailing ‘0’ ‘0023’.rstrip(‘0’) Remove leading ‘0’ ‘0023’.lstrip(‘0’) Trip a string ’ Hello ‘.strip() Find location of substring ‘abc’.find(‘d’)= (returns -1) Find location of substring ‘abc’.index(‘d’)= (raise exception) Check whether substring “el” in “hello world” Replace string ‘ab cd’.replace(’',”) Padding leading zero ‘101’.zfill(10) Padding whitespace to the left ‘a’.ljust(10,’=’) Padding whitespace to the right ‘a’.rjust(10,’=’) Format string “%s,%d,%s” % (“2012”, 12, “12”)
Name Comment Error: i++ OK: i += 1 Error: b=true OK: b=True Error: i<len(A) && j<len(B): OK: i<len(A) and j<len(B): Error: for i>=0 and j>=0: OK: while i>=0 and j>=0: Error: ! f OK: not f NameError: name ‘List’ is not defined from typing import ListPython float with high resolution
1.8 Pip - Python Package Management
Name Comment Check on installed python package pip show simplejson Search a package pip search simplejson Install and uninstall a package pip install simplejson, pip uninstall simplejonInstall package with a specific version pip install flake8==2.0 Show installation folder of a module module.__file__, flask.__file__Check on-line help for a module help(module)pip install -U simplejon pip install -i http://pypi.python.jp flask
Name Comment max, min sys.maxsize, -sys.maxsize-1min, max min(2, 3), max(5, 6, 2)min with customized comparision min(a, b, key=lambda x: x*x-2*x+1)generate range for num in range(10,20)get ascii ord('a'), chr(97)print integer in binary “{0:b}”.format(10)
Name Comment dict get first element m[m.keys()[0]]get by key with default value m.get(x, -1)Dictionary with defaults m = collections.defaultdict(lambda: 1)Dictionary with tuple defaults d=collections.defaultdict(lambda: (0, 0))), d[0, 1]=(2, 3) Use array as key in dictionary Convert array to tuple: m[tuple(l)]=3 Check whether key in hashmap if k in mLoop dictionary by keys for k in mLoop dictionary for k,v in m.items(), not for k,v in enumerate(m) Intersection of two sets list(set(l1).intersection(set(l2)))List to set set(list1)Remove from set s.remove(2)Deep copy dict import copy; m2=copy.deepcopy(m1)Remove the first from set s.pop()Sort dict by values sorted(dict1, key=dict1.get)Convert a str to a dict eval("{\"createtime\":\"2013-07-16\"}")Delete an element from a dict del d[key]
Name Comment mod x % 2shift left x << 1; a << 2shift righ x >> 2and x & ycomplement ~xxor x ^ ypower 2 ** 3bool complement not xbinary format bin(5) (get 101)count 1 inside binary bin(5).count('1')
Name Comment Append file open("/tmp/test.txt", "ab").write("\ntest:")Write file open("/tmp/test.txt", "wab").write("\ntest:")Read files f.readlines()Check file os.path.exists("/tmp/test.txt")Reference Github: cheatsheet-python-A4/code/exampleFile.py
Name Comment sqrt import math; math.sqrt(5)power import math; math.pow(2, 3)log import math; math.log(5, 2), log2(5)random random.randint(1, 10) 1 and 10 includedeval string eval("2-11*2")
Name Comment Send http REST call pip install requests; r = requests.get(‘https://XX/XX’, auth=(‘user’, ‘pass’)) Start a simple HTTP server python -m SimpleHTTPServer <port_number>
Name Comment Initialize min heap heapq.heapify(q)heappush a tuple q=[]; heapq.heappush(q, (5, ‘ab’)) pop print (heapq.heappop(q))first item q[0]print heapq print list(q)create a queue from collections import deque; queue = deque([1,5,8,9])append queue queue.append(7)pop queue from head element = queue.popleft()Reference Link: Python Heapq
import heapq
# initializing list
li = [5 , 7 , 9 , 1 , 3 ]
# using heapify to convert list into heap
heapq .heapify (li ) # a minheap
heapq ._heapify_max (li ) # for a maxheap!
# printing created heap
print (list (li ))
# using heappush() to push elements into heap
# pushes 4
heapq .heappush (li ,4 )
# printing modified heap
print (list (li ))
# using heappop() to pop smallest element
print (heapq .heappop (li ))
print (list (li ))
Initialize Linkedlist from array
def initListNodeFromArray (self , nums ):
head = ListNode (None )
prev , p = head , head
for num in nums :
pre = p
p .val = num
q = ListNode (None )
p .next = q
p = p .next
pre .next = None
return head def printListNode (self , head ):
print ("printListnode" )
while head :
print ("%d" % (head .val ))
head = head .next
Print Trie Tree in level order
def printTrieTreeLevelOrder (self , node ):
print ("printTrieTreeLevelOrder" )
if node .is_word :
print ("Node is a word" )
queue = []
queue .append (node )
while len (queue ) != 0 :
s = ''
for i in range (len (queue )):
node = queue [0 ]
del queue [0 ]
for child_key in node .children :
s = '%s %s' % (s , child_key )
queue .append (node .children [child_key ])
if s != '' :
print 'print level children: %s' % (s )
python sort with customized cmp function: -1 first
nums = [3 , 2 , 6 ]
def myCompare (v1 , v2 ):
return - 1
sorted_nums = sorted (nums , cmp = myCompare )
print nums # [3, 2, 6]
print sorted_nums # [6, 3, 2] col_count , row_count = 3 , 2
matrix = [[None for j in range (col_count )] for i in range (row_count )]
print matrix 1.18 Python Common Algorithms
Num Category/Tag Example 1 #bfs Leetcode: Max Area of Island 2 #dfs LeetCode: Surrounded Regions 3 #binarysearch LeetCode: Search Insert Position 4 #interval , #mergelist LeetCode: Interval List Intersections 5 #twopointer , #array LeetCode: Reverse Words in a String II 6 #twopointer LeetCode: Two Sum 7 #backtracking , #subset LeetCode: Subsets II 8 #linkedlist , #presum LeetCode: Remove Zero Sum Consecutive Nodes from Linked List 9 #unionfind LeetCode: Accounts Merge 10 #trie LeetCode: Longest Word in Dictionary 11 #stack LeetCode: Valid Parentheses 12 #stack LeetCode: Reverse Substrings Between Each Pair of Parentheses 13 #heap LeetCode: Top K Frequent Elements 14 #baseconversion LeetCode: Base 7 , LeetCode: Convert to Base -2 15 #interval LeetCode: Meeting Rooms II , LeetCode: My Calendar I 16 #monotone LeetCode: Daily Temperatures 17 #knapsack LeetCode: Coin Change 18 #sortbyfunction LeetCode: Relative Sort Array 19 #slidingwindow LeetCode: Longest Substring Without Repeating Characters 20 #editdistance , #dynamicprogramming LeetCode: Longest Common Subsequence 21 #twopointer , #mergetwolist LeetCode: Merge Sorted Array 22 #topologicalsort LeetCode: Course Schedule 23 #bfs , bidirectional bfs LeetCode: Word Ladder 24 #monotonicfunc , #binarysearch LeetCode: Kth Smallest Number in Multiplication Table 25 #divideconquer , #recursive Leetcode: Count of Smaller Numbers After Self 26 python semaphore LeetCode: Print Zero Even Odd
License: Code is licensed under MIT License .
https://www.tutorialspoint.com/python_data_structure/index.htm
