76. Minimum Window Substring

 def minWindow(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: str
        """
        counter = collections.Counter(t)
        count, start, end, head, res = len(t), 0, 0, 0, float('inf')
        while end < len(s):
            if counter[s[end]] > 0:
                count -= 1
            counter[s[end]] -= 1
            end += 1
            while count == 0:
                if end - start < res:
                    res = end - start
                    head = start
                if counter[s[start]] == 0:
                    count += 1
                counter[s[start]] += 1
                start += 1
        return '' if res == float('inf') else s[head:head + res]