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.

p1, p2 表示某个char最后一次出现的地方.longest substring可能只包含一种char, 或者两种char.1. 只包含一种Char, 强制保持p1 == p22. 出现两种char, 保证p1 <= p2， 为了计算方便3. 出现第三种char的时候，i - (p1+1) + 1 = i- p1 直接计算出当前substring长度。同时保证p1<=p2.

public class Solution {    public int lengthOfLongestSubstringTwoDistinct(String s) {        if(s == null || s.length() == 0) return 0;        int p1 = 0, p2 = 0, len = 1, max = 1;        char[] arr = s.toCharArray();        for(int i = 1; i < arr.length; i++){            // third char appear            if(p1 != p2 && arr[i] != arr[p1] && arr[i] != arr[p2]){                if(len > max) max = len;                len = i - p1;                p1 = p2;                p2 = i;            } else {                // same char as p1 and p2                if(arr[i] == arr[p1]){                    p1 = p1 == p2 ? i : p2;                }                len++;                p2 = i;            }        }        if(len > max) max = len;        return max;    }}