Given an integer``` k``` and a string ```s```, find the length of the longest substring that contains at most k distinct characters. For example, given ```s = "abcba" ```and ```k = 2```, the longest substring with k distinct characters is``` "bcb"```.
Given an integer
kand a strings, find the length of the longest substring that contains at most k distinct characters.For example, given
s = "abcba"andk = 2, the longest substring with k distinct characters is"bcb".