Longest Substring with At Most Two Distinct

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.

用p1 & p2 两个pointer分别纪录当前window里两个character最后一次发生时的index，用start纪录window开始时的index。
从index 0开始遍历String:

如果当前的character在window内，update相应的pointer。

如果不在，比较两个character的pointer，去掉出现较早的那个character, 更新start=min(p1,p2)+1

时间复杂度是O(n), 空间复杂度是O(1):

 public class Solution {
     public int lengthOfLongestSubstringTwoDistinct(String s) {
         int result = 0;
         int first = -1, second = -1;
         int win_start = 0;
         for(int i = 0; i < s.length(); i ++){
             if(first < 0 || s.charAt(first) == s.charAt(i)) first = i;
             else if(second < 0 || s.charAt(second) == s.charAt(i)) second = i;
             else{
                 int min = first < second ?  first : second;
                 win_start = min + 1;
                 if(first == min) first = i;
                 else second = i;
             }
             result = Math.max(result, i - win_start + 1);
         }
         return result;
     }
 }