原题链接在这里：

题目：

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.

题解：

与类似。快慢指针维护substring的方法在里有总结.

runner 扫过 char的frequency加一. 条件count++.

当count > 2后移动walker直到count减到2

Time Complexity: O(n), 窗口只移动一遍. Space: O(1). map size.

AC Java:

1 public class Solution { 2     public int lengthOfLongestSubstringTwoDistinct(String s) { 3         int res = 0; 4         int [] map = new int[256]; 5         int walker = 0; 6         int runner = 0; 7         int count = 0; 8         while(runner < s.length()){ 9             if(map[s.charAt(runner++)]++ == 0){10                 count++;11             }12             while(count > 2){13                 if(map[s.charAt(walker++)]-- == 1){14                     count--;15                 }16             }17             res = Math.max(res, runner - walker);18         }19         return res;20     }21 }

类似.