[Leetcode]Longest Substring with At Most Two Distinct Characters

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

For example,
Given S = “eceba”,

T is "ece" which its length is 3. 

给定一个字符串S，找到最长的子字符串T使得T里最多含有2个不同的字母~

设定数组count，储存字母出现的次数；numOfDistinct 记录当前子字符串不同字母的个数~代码如下~

def longestSubstring(s):
	if s is None or len(s) == 0: return 0
	count = [0] * 256
	i = 0; numOfDistinct = 0; maxLen = 0
	for j in xrange(len(s)):
		if count[s[j]]  == 0: numOfDistinct += 1
		count[s[j]] += 1
		while numOfDistinct > 2:
			count[s[i]] -= 1
			if count[s[i]] == 0: numOfDistinct -= 1
			i += 1
		maxLen = max(maxLen, j - i + 1)
	return maxLen