Problem

Given a string s and an integer k, determine the maximum length of a substring where all characters are the same after modifying up to k characters.

You can replace any character in the string with another uppercase English character to achieve this.

Examples

maxSubstring("AAABBC", 2)
// return 5
// Why? You can form "AAAAAC" by replacing two Bs

maxSubstring("ABCDE", 0)
// return 1
/* Why? With no changes allowed (k = 0),
longest substring with repeating characters is 1 */