Longest Substring with At Most K Distinct Characters

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

public class Solution {
  public int lengthOfLongestSubstringKDistinct(String s, int k) {
        if (s == null || s.length() == 0 || k <= 0) return 0;

        Map<Character, Integer> map = new HashMap<>();
        int left = 0;

        int result = 0;
        for (int i = 0; i < s.length(); i++) {
            map.merge(s.charAt(i), 1, (a, b) -> a + b);

            if (map.size() > k) {
                while (map.size() > k) {
                    map.merge(s.charAt(left), 1, (a,b)->a-b);
                    if (map.get(s.charAt(left)) == 0) {
                        map.remove(s.charAt(left));
                    }

                    left++;
                }
            }

            result = Math.max(result, i - left+1);
        }

        return result;
    }
}