LeetCode-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.

Analysis:

Adding ith char into current substring, keep tracking the number of each char in the current substring, if the distinct number constraint is violated, we move the start forward until the contraint is satisfied.

Solution:

public class Solution {
    public int lengthOfLongestSubstringKDistinct(String s, int k) {
        if (s.isEmpty() || k==0) return 0;
        
        HashMap<Character,Integer> charMap = new HashMap<Character,Integer>();
        int start = 0, end = 1, distNum = 1;
        charMap.put(s.charAt(0),1);
        
        int res = 1;
        while (end<s.length()){
            char cEnd = s.charAt(end);
            if (charMap.containsKey(cEnd)){
                charMap.put(cEnd,charMap.get(cEnd)+1);
            } else {
                charMap.put(cEnd,1);
                distNum++;
                while (distNum>k){
                    char cStart = s.charAt(start);
                    int val = charMap.get(cStart)-1;
                    if (val==0){
                      distNum--;
                      charMap.remove(cStart);
                    } else charMap.put(cStart,val);
                    start++;
                }
            }
            end++;
            res = Math.max(end-start,res);
        }
        return res;
    }
}