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Code snippets (CPP version)

a wise man once said

CPP means competitive programming in Python :D

Some code snippets in Python that I concluded so that I can do LeetCode faster.

Input & output

In some platforms like Codeforces or UVA, sometimes reading input is complicated if you're not familiar with it.

Read integer input

Read an integer in a line:

n = int(input())

Read many integers in a line:

9, 10

na,nb = map(int, input().split())

> na: 9, nb: 10

Read many lines into an array, use list(map(int, input().split())):

2 4
5 6

a = []
for i in range(2):
a.extend(list(map(int,input().split())))

> a: [2, 4, 5, 6]

Built-in functions

Python has many things that has built up for us. Why not using them?

Max, min

Use max, min for faster finding:

current_max = max(current_max, array[i])
current_min = min(current_min, array[i])

Sorting

Use sort to sort array (mutable). The complexity is usually O(NLOGN)O(NLOGN).

arr.sort()

Use sorted for immutable:

sorted_arr = sorted(arr)

Using lambda

Lambda function is an anonymous function with 1 expression. You can prefer something like (x) => x + 1 in JavaScript for example.

x = lambda a : a + 10
print(x(5)) # 15

You can use lambda function with map, filter, etc.

Note that map returns map object so we have to convert it to array using list()

arr = [1, 2, 3, 4, 5]
list(map(lambda n: n * 2, arr))

> [2, 4, 6, 8, 10]

One example that we want addition of arrays. For example, [1, 2, 1, 0] plus [0, 1, 2, 1] like this:

 1 2 1 0
0 1 2 1
1 3 3 1 <-

Simply add is not going to work:

sum = [1, 2, 1, 0] + [0, 1, 2, 1]

> sum: [1, 2, 1, 0, 0, 1, 2, 1]

So we use lambda function with map() and convert it using list():

arr1 = [1, 2, 1, 0]
arr2 = [0, 1, 2, 1]
sum = list(map(lambda x, y: x + y, arr1, arr2))

> sum: [1, 3, 3, 1]
Example:

Array tips

Declare 2-dimension array

Instead of:

2d_arr = [0] * rows
for i in range(rows):
res[i] = [0] * cols

# for example with rows = 3, cols = 4:
[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
0 0 0 0
0 0 0 0
0 0 0 0

Do these:

2d_arr = [[0 for _ in range(cols)] for _ in range(rows)]

Note that we don't use arr = [] * n, cause this is not create anything. So we had to have initial value for array.

Declare

When we do things related to 2-D array, remember divide (//) and module (%).

Examples

Data structures

Set

Counter

When counter problems that requires counting elements repeat, etc. Example is the problem that ask for repeat of each character in a string.

We can use Counter from collections library, this data structure creates a dictionary that has element is key, value is count variable.

from collections import Counter

# create dictionary stores <objects,counts> as <key,value>
cnt = Counter('hello world')

cnt['o'] # 2, count of character 'c'
cnt['o'] += 1

Heap