8.10 - hard - 37

159. Longest Substring with At Most Two Distinct Characters

这题比较简单，前向型指针的模板套用一下就可以了

class Solution(object):
    def lengthOfLongestSubstringTwoDistinct(self, s):
        """
        :type s: str
        :rtype: int
        """
        # 前向型指针
        j = 0
        max_length = 0
        h = {}
        for i in range(len(s)):
            
            while j < len(s) and self.valid(h, s, j):
                h[s[j]] = h.get(s[j], 0) + 1 
                max_length = max(max_length, j - i + 1)
                j += 1
            
            h[s[i]] -= 1
            if h[s[i]] == 0:
                h.pop(s[i])
        
        return max_length

    def valid(self, h, s, j):
        if len(h) <= 1:
            return True
        return s[j] in h