Longest Repeating Character Replacement
You are given a string s and an integer k. You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most k times.

Return the length of the longest substring containing the same letter you can get after performing the above operations.

Example 1:
Input: s = "ABAB", k = 2
Output: 4
Explanation: Replace the two 'A's with two 'B's or vice versa.

Example 2:
Input: s = "AABABBA", k = 1
Output: 4
Explanation: Replace the one 'A' in the middle with 'B' and form "AABBBBA".
The substring "BBBB" has the longest repeating letters, which is 4.

Possible solution: Simple and Naive
Generate all substrings
    abc
--
a
ab
abc
b
bc
c

Use HashMap to count the number of characters in each substring
    banana
------
b -> 1
a -> 3
n -> 2

The maximum number of characters is 'a' => 3
len(banana) = 6
6 - 3 = 3
The number of changes is 6 - 3 = 3 , k = 3

Generate all substrings of banana
The total number of substrings are (6, 1) + (6, 2) + (6, 3) + (6, 4) + (6, 5) + (6, 6) = 2^6 - 1  (no empty string)

Iterate through all substrings and find the maximum of character count

ana
a -> 2
n -> 1

m = len(ana) - 2 = 1
if (m == k)
then get the length of ana which is 3

Source Code
    \$b/java/LongestRepeatingCharRep.java