Leetcode-Longest Substring with At Most Two Distinct Characters.

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.

Solution:

public class Solution {
    public int lengthOfLongestSubstringTwoDistinct(String s) {
        //Two pointers: start, end. Maintain that start-end is a valid substring.
        //Record the number of the two chars in start-end.
        //Inc end, if the charAt(end) is a thrid char, then inc start, until the start-end is a valid substring again
        //record the length of the new string.

        Map<Character,Integer> charNum = new HashMap<Character,Integer>();
        int start = 0;
        int end = 0;
        int charType = 2;
        int maxLen = 0;
        while (end<s.length()){
            char cur = s.charAt(end);
            //if this char is in substring already, then increase its number
            if (charNum.containsKey(cur))
                charNum.put(cur,charNum.get(cur)+1);
            else {
                charNum.put(cur,1);
                if (charType>0) charType--;
                else {
                   //We need eliminate another char in substring to maintain the feasibility of the substring.
                   while (charNum.get(s.charAt(start))>1){
                       charNum.put(s.charAt(start),charNum.get(s.charAt(start))-1);
                       start++;
                   }
                   charNum.remove(s.charAt(start));
                   start++;
                }
            }

            if (maxLen<end-start+1) maxLen = end-start+1;
            end++;
        }

        return maxLen;        
    }
}