Longest Substring with At Most Two Distinct Characters(Hard)

Given a string, find the length of the longest substring T that contains at most 2 distinct characters.

For example, Given s = “eceba”,

T is "ece" which its length is 3.

Java代码

public static String maxSubStringKUniqueChars(String s, int k) {
    //declare a counter
    HashMap<Character, Integer> map = new HashMap<Character, Integer>();        
    int start = 0;
    int maxLen = 0;
    String maxSubstring = null;

    for (int i = 0; i < s.length(); i++) {
        //add each char to the counter
        char c = s.charAt(i);
        if(map.containsKey(c)){
            map.put(c, map.get(c)+1);
        }else{
            map.put(c, 1);
        }

        if(map.size() == k+1){
            //get maximum
            int range = i-start;
            if(range > maxLen){
                maxLen = range;
                maxSubstring = s.substring(start, i);
            }

            //move left cursor toward right, so that substring contains only k chars
            while(map.size()>k){
                char first = s.charAt(start);
                int count = map.get(first);
                if(count>1){
                    map.put(first,count-1);
                }else{
                    map.remove(first);
                }
                start++;
            }
        }
    }

    if (map.size() == k && maxLen == 0) {
        return s;
    }

    return maxSubstring;
}