Medium
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.

刷到谷歌面经“http://practice.geeksforgeeks.org/problems/longest-k-unique-characters-substring/0”这道题时发现的类似题，几乎就是一模一样，唯一的区别是这里多了个at most.也就是当s.length() < k时，返回的不是-1而是s.length().

class Solution {
    public int lengthOfLongestSubstringKDistinct(String s, int k) {
        if (s == null || s.length() == 0 || s.length() < k){
            return s.length();
        }
        char[] chars = s.toCharArray();
        Map<Character, Integer> map = new HashMap<>();
        int start = 0;
        int end = 0;
        int maxLen = 0;
        for (; end < chars.length; end++){
            map.put(chars[end], map.getOrDefault(chars[end], 0) + 1);
            while (map.size() > k){
                char c = chars[start];
                map.put(c, map.get(c) - 1);
                if (map.get(c) == 0){
                    map.remove(c);
                }
                start++;
            }
            maxLen = Math.max(maxLen, end - start + 1);
        }
        return maxLen;
    }
}