滑动窗口+哈希

# 
# @param S string字符串 
# @param T string字符串 
# @return string字符串
#
class Solution:
    def minWindow(self , S: str, T: str) -> str:
        # 维护两个哈希表，hashT表示需要的字符，hash1表示记录窗口内满足条件的字符
        hashT = {}
        for i in list(T):
            if i not in hashT: 
                hashT[i] = 1
            else:
                hashT[i] += 1
        # 一次遍历找“可行解”
        res = ''
        for i in range(len(S)):
            # hash1 = hashT 默认深拷贝，等价于hash1 = hashT.deepcopy()
            hash1 = hashT.copy()
            if S[i] not in T:
                continue
            else:
                # 右窗口移动
                for j in range(i,len(S)):
                    if S[j] in hash1:
                        if hash1[S[j]] == 1:
                            del hash1[S[j]]
                        else:
                            hash1[S[j]] -= 1
                     # 判断窗口是否已包含所有子串
                    if len(hash1) == 0:
                        if res == '':
                            res = S[i:j+1]
                        elif j-i+1 < len(res):
                            res = S[i:j+1]
                        break
        return res