Given a string, find the length of the longest substring T that contains at most k distinct characters.
For example, Given s = “eceba” and k = 2,
T is "ece" which its length is 3.
I used "shakutoriho". Firstly, I check the 's' string from the head to the end with right iterator. I can save the information whether I found the character in hash map. When I found the new character, I increment the count of the distinct numbers. If I found the character which I already had found, I can proceed the left iterator and decrement the count of the distinct numbers. Through this manipulation, if the situation is max numbers of the distinct num > k, I can save the max difference between right iterator and left iterator. It means the max length substring I wanna get.