sfdye / python-for-leetcode

python tricks that will help you with leetcode

python-for-leetcode

Python tricks that will help you with leetcode

Tricks

1. Use Python 3

2. One-liner Trie

trie = lambda :defaultdict(trie)
trie = trie()

Implement trie prefix tree

3. Flatten 2D array (matrix)

a = [
  [1,2,3],
  [4,5,6],
  [7,8,9]
]

print(sum(a), [])

4. Reverse a list or string

str = "abcde"
print(str[::-1])

a = [1, 2, 3, 4, 5]
print(a[::-1])

5: Array index trick (e.g. Palindrome checking)

# Before
str = "abcba"
n = len(str)
print(all(str[i] == str[n-i-1] for i in range(n)))

# After
str = "abcba"
print(all(str[i] == str[~i] for i in range(len(str))))

6. Tuple unpacking

7. Boolean as int

8. map

9. redeuce

10. Copy list

a = [1, 2, 3]
b = a[:]

11. Dictionary defualt value

# Before
d = {}  # dictionary of list
if k in d:
    d[k].append(v)
else:
    d[k] = [v]

# After

# 1. setdefault method on dict
d = {}  # dictionary of list
d.setdefault(k, []).append(v)

# 2. collections.defaultdict
d = collections.defaultdict(list)
d[k].append(v)

12. Swap variables

a, b = b,a

13. Chained boolean comparisons

if 1 < 2 < 3:

if 'a' < 'c' > 'b':

if 1 == True != False:

Todo:

• zip
• float("inf")
• itertools.zip_longest
• collections.Counter
• heapq
• list as queue
• collections.deque as stack
• string.split
• itertools.chain
• yield in recursion
• sort vs sorted
• sort tuple, sort by key
• range vs xrange
• string.join
• bisect
• list initialzation (using asterisk and anti-pattern)
• enumerate
• Fix markdown syntax
• Add example problems to each trick

Reference

• https://github.com/brennerm/PyTricks
• Leetcode problem discussions (especially comments from Stefan Pochmann)