In what concerns the continuous evaluation solving exercises grade during the semester, you should submit until 23:59 of November 22nd
(this exercise will still be available for submission after that deadline, but without counting towards your grade)
[to understand the context of this problem, you should read the class #06 exercise sheet]

In this problem you should submit a function as described. Inside the function do not print anything that was not asked!

The name of the .py source file you submit to Mooshak should NOT start with a digit (you will get an error if you submit it like that).

[IP066] Alphabetical Obsession

Sheldon Cooper has developed a new obsession: he only wants to read text segments that are "letter-controlled". In any piece of text he examines, there can be at most k different letters. Everything else, such as digits, punctuation or spaces, doesn't bother him, but letters are sacred, and too many distinct ones make him anxious.

One day, Sheldon found a long string of text on the whiteboard in the university cafeteria. He wants to figure out the longest contiguous segment he can read without exceeding his distinct-letter limit, but he needs your help.

The Problem

Write a function letters(k, str) that receives a receives an integer k and a string str and returns the length of the largest substring (a contiguous sequence of characters from the original string) that contains at most k different letters.

All letters in the string are guaranteed to be in lowercase. Non-letter characters count towards the length to be computed, but do not count as letters towards the limit of at most k distinct letters.

Constraints

The following limits are guaranteed in all the test cases that will be given to your program:

NOTE: the limits are high and you must have an efficient solution.
If your code checks all possible segments and iterates in each one through all characters one by one, it will result in a Time Limit Exceeded.

print(letters(2, "sheldon cooper"))
print(letters(6, "introduction to programming"))
print(letters(4, "ip|cc1024|dcc|fcup"))

4
11
15

Explanation of the first example

The largest substring is " coo" with length=4 and 2 distinct letters (c and o)

Explanation of the second example

The largest substring is "tion to pro" with length=11 and 6 distinct letters (t, i, o, n, p and r)

Explanation of the third example

The largest substring is "p|cc1024|dcc|fc" with length=15 and 4 distinct letters (p, c, d and f)