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1.1 Python Compact Coding
Name Comment if … return if k == 0: return False if… continue if index == icol: continue 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]*3sum 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)%5
1.2 Python Common Algorithms
Name Comment return all but last list[:-1]The second last item list[-2] or list[~1]map map(lambda x: str(x), [1, 2, 3])create fixed size array l = [None] * 5insert elements to head array.insert(0,var)delete element by index del a[1]list as stack item = l.pop()sort in descending l = sorted([8, 2, 5], reverse=True) sort by attribute l=sorted([(‘ebb’,12),(‘abc’,14)], key=lambda x: x[1]) in-place sort l.sort()generate a-z map(chr, range(ord('a'), ord('z')+1))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]
Name Comment reverse string ‘hello world’[::-1] array to string ’ ‘.join([‘a’, ‘b’]) split string to array “hello, python”.split(“,”) string to array list('abc')format to 2 digits print "%02d" % (13)find location of substring ‘abc’.find(‘d’)= (returns -1) find location of substring ‘abc’.index(‘d’)= (raise exception) capitalize string ‘hello world’.capitalize() upper/lower string ‘aBc’.upper()=, ‘aBc’.lower() count substring ‘2-5g-3-J’.count(‘-‘) replace string ‘ab cd’.replace(’ ‘, ”) padd whitespace to the left ‘a’.ljust(10, ’ ‘) padd whitespace to the right ‘a’.rjust(10, ’ ‘) pad leading zero ‘101’.zfill(10) string remove tailing ‘0’ ‘0023’.rstrip(‘0’) string remove leading ‘0’ ‘0023’.lstrip(‘0’) check if string represent integer ‘123’.isdigit() check if string alphabetic ‘aBc’.isalpha() Check if string alphanumeric ‘a1b’.isalnum()
Name Comment max, min sys.maxsize, -sys.maxsize-1min, max min(2, 3), max(5, 6, 2)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]]intersection list(set(l1).intersection(set(l2)))list to set set(list1)remove from set s.remove(2)remove the first from set s.pop()sort dict by values sorted(dict1, key=dict1.get)deep copy dict import copy; m2=copy.deepcopy(m1)
Name Comment mod x % 2shift left x << 1 ; a << 2=shift 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")
Name Comment sqrt import math; math.sqrt(5)power import math; math.pow(2, 3)random random.randint(1, 10) 1 and 10 includedeval string eval("2-11*2")
Name Comment 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()
Review: Heap Problems
Link: BINARY HEAP AND HEAPQ IN PYTHON
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 License: Code is licensed under MIT License .
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