Odessa PyConerence 2018

1. Competitive
Programming
using Python
Andrii Soldatenko
29 September 2018
OdessaPY ‘18
@a_soldatenko

2. Andrii Soldatenko
• Contributor in OpenAPI, Golang,
Pyhelm, python-sendgrid, tutorial of
aiohttp, ansible, requests, etc.
• Speaker at many PyCons,
committee member of PyCon
Belarus and PyCon Russia and
open source contributor
• blogger at https://asoldatenko.com
@a_soldatenko

5. Edsger Dijkstra
1930-2002

6. Given well-known
Computer Science
problems
@a_soldatenko
https://en.wikipedia.org/wiki/
List_of_unsolved_problems_in_computer_science

7. @a_soldatenko
Disclamer

8. @a_soldatenko
Anatomy of a Programming
Contest Problem
• Background story / Summary
• Input and Output description
• Sample Input/Output
• Hints/Pictures

9. @a_soldatenko
How it looks like?

11. top500 ~500 problem solved

12. 0
5000
10000
15000
20000
25000
30000
2008 2009 2010 2011 2012 2013 2014 2015 2016 2017
Number of contestants used
Python compare to other
languages
C C++ Java Python

13. Tip 1:
Type code faster
https://www.typingtest.com/

14. https://www.typingtest.com/

16. Tip 2:
Do Algorithm analysis

17. Time complexity
O( . . . )

18. for i in range(n):
pass

19. for i in range(n):
pass
O(n)

20. for i in range(n//2):
pass

21. for i in range(n//2):
pass
O(n)

22. for i in range(n+2):
pass

23. for i in range(n+2):
pass
O(n)

24. for i in range(n):
for j in range(n):

25. for i in range(n):
for j in range(n):
O(n2
)

26. def foo(n):
if n == 1: return
foo(n-1)
foo(n-1)

27. function call number of calls
foo(n) 1
foo(n-1) 2
…. …
foo(1)

28. function call number of calls
foo(n) 1
foo(n-1) 2
…. …
foo(1) 2n
− 1

29. Time Complexity
CPython -> list
Append[1] -> O(1)
Get Item -> O(1)
Sort -> O(n log n)
https://wiki.python.org/moin/TimeComplexity

30. Tip 3:
Identify
problem types

31. Problem types:
• Ad hoc
• Complete Search (Iterative/Recursive)
• Divide and Conquer
• Dynamic Programming

32. Problem types:
• String Processing
• Computational Geometry
• Graph
• Mathematics

33. @a_soldatenko
Tip 5:
Master Your
Programming Languages

34. @a_soldatenko

35. https://docs.python.org/3/reference/index.html

36. Tip 6:

37. @a_soldatenko
Input / Output Routines:
Sample input:
3
1 2
5 7
6 3

38. @a_soldatenko
Input / Output Routines:
input()
sys.stdin
open(‘sample.in’)

39. @a_soldatenko
Please don’t use prints
for debugging

40. $ python main.py < sample.in
> /Users/andrii/fan/fan/uva_answers/000/
main.py(6)main()
5 import ipdb; ipdb.set_trace()
----> 6 for line in sys.stdin:
7 print(line)
ipdb> *** SyntaxError: invalid syntax
ipdb> *** SyntaxError: invalid syntax

41. @a_soldatenko
Simple tip:
def fopen(filename, default=sys.stdin):
try:
f = open(filename)
except FileNotFoundError:
f = default
return f

42. @a_soldatenko
Simple tip:
$ python main.py
> /Users/andrii/fan/fan/uva_answers/000/
main.py(13)main()
12 import ipdb; ipdb.set_trace()
---> 13 for line in fopen('sample.in'):
14 print(line)

43. Let’s solve problems

44. 195 - Anagram
Given the word “abc”, your
program should — by exploring
all different combination of the
three letters — output the
words “abc”, “acb”, “bac”,
“bca”, “cab” and “cba”.
https://uva.onlinejudge.org/external/1/195.pdf

45. int main()
{
string s = "aba";
sort(s.begin(), s.end());
do {
cout << s << 'n';
} while(next_permutation(s.begin(),
s.end()));
}
// aab
// aba
// baa
@a_soldatenko

46. from itertools import permutations
def main():
for perm in permutations('aba'):
print(''.join(perm))

47. # aab
# aab
# aba
# aba
# baa
# baa
from itertools import permutations
def main():
for perm in permutations('aba'):
print(''.join(perm))

48. https://docs.python.org/3/library/
itertools.html#itertools.permutations

49. Serhiy Storchaka proposed
in python-ideas to add:
• factorial(n)
• gcd(n, m)
• as_integer_ration(x)
• binom(n, k)
• isprime(n)
• primes()

50. @a_soldatenko

51. Telegram channel
http://t.me/
python_programming_challenges

52. Thank You
andrii.soldatenko@toptal.com
@a_soldatenko

53. Questions
? @a_soldatenko

54. Telegram channel
http://t.me/
python_programming_challenges