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.

Solution：滑动窗

思路：
Time Complexity: O(N) Space Complexity: O(1)

Solution Code：

class Solution {
    public class Solution {
        public int lengthOfLongestSubstringKDistinct(String s, int k) {
            int[] count = new int[256];
            int num = 0, i = 0, res = 0;
            for (int j = 0; j < s.length(); j++) {
                if (count[s.charAt(j)]++ == 0) num++;
                if (num > k) {
                    while (--count[s.charAt(i++)] > 0);
                    num--;
                }
                res = Math.max(res, j - i + 1);
            }
            return res;
        }
    }
}