MyNotes.md

Python Note Taking

#zip example
>>> x = [1, 2, 3]
>>> y = [4, 5, 6]
>>> zipped = zip(x, y)
>>> zipped
[(1, 4), (2, 5), (3, 6)]
>>> x2, y2 = zip(*zipped)
>>> x == list(x2) and y == list(y2)
True

a = [(1,2,3),(8,9,7),(199,88,10)]

b = [n for nd in a for n in nd] # for nd in a (tuples)
                                # for n in nd (everything in each tuples)
print b
>>>[1,2,3,8,9,7,199,88,10]

#copy 1D array by value
b = a[:]
#copy 2D array by value
b = [c[:] for c in a]

a = [[1,2,0],[4,5,6]]
print len(a)
print len(a[0])
>>> 2
>>> 3

#map example Pi
result = map(lambda x: x * x, [1, 2, 3])
reuslt = [1, 4, 9]

#filter example
result = filter(lambda x: x > 0, [-2, -1, 0, 1, 2])
reuslt = [1, 2]

#reduce example
result = reduce(lambda x, y: x + y, [1, 2, 3, 4])
result = 10

a = [1, 2, 3, 4]
print a[::-1]
>> [4, 3, 2, 1]

b = [[1,2],[3,4]]
print b[::-1]
>> [[3, 4], [1, 2]]

maxInt = 2 ** 31 -1
minInt = -2 ** 31

d = dict()
d.pop(somekey, None) #remove key from dict
len(d)
d.keys() # [key1, key2, ...] all keys of a dict
d.values() #[value1, value2....] all values of a dict

(set(range(11)) - set(range(10))).pop()

>> 10

More about sets

num = ['99', '200','93']
num.sort(lambda x, y: cmp(y+x, x+y))

>> num = ['99', '93', '200']
>> ''.join(num).lstrip('0') or '0'
# join ---->  9993200
# lstrip ----> get rid of all 00000 prefix  (avoid ['0', '0'])
# or '0'  -----> '' or '0' = '0'

>>> mylist = ['a', 'b', 'c', 'd']
>>> [item for item in enumerate(mylist)]
[(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd')]

import collections
a = [1,2,3,4,5,5,5]
c = collections.Counter(a)
print c
print c[5]
>>>Counter({5: 3, 1: 1, 2: 1, 3: 1, 4: 1})
>>>3

Stacks & Queue

Stacks: LIFO: Last in, first out Queue: FIFO: First in, first out

Stacks

Push: add item to top of the stacks Pop: remove item from top of the stacks peek: return top item without removing

Queue

enqueue:add item to the back of the queue dequeue:remove item from front of the queue peek: return front item

Linked List

next prev singly linked — omit the prev insert in the head

Binary Search Tree

left sub smaller or eq right sub bigger http://www.code123.cc/docs/leetcode-notes/basics_data_structure/binary_tree.html