最小覆盖子串

class Solution:
    def minWindow(self, s, t):
        res = ""
        if len(s) < len(t):
            return res    
        left = 0
        right = 0
        min_len = len(s) + 1
        m = {}
        count = 0
        for i in t:
            m[i] = m.get(i,0) + 1#统计t中字符数目
        while right < len(s):
            if s[right] in m:
                m[s[right]] -= 1
                if m[s[right]] >= 0:
                    count += 1
                while (count == len(t)):
                    if (right - left + 1 < min_len):
                        min_len = right-left+1
                        res = s[left:right+1]
                    if s[left] in m:
                        m[s[left]] += 1
                        if m[s[left]] > 0:
                            count -= 1
                    left += 1
            right += 1      
        return res