from collections import defaultdict

class Solution:

def minWindow(self, s: str, t: str) -> str:

char_frequency = defaultdict(lambda: 0)

for c in t:

char_frequency[c] += 1

chars_matched = 0

start = 0

res = ""

for end in range(len(s)):

right_char = s[end]

if right_char in t:

char_frequency[right_char] -= 1

if char_frequency[right_char] == 0:

chars_matched += 1

while start <= end and chars_matched == len(char_frequency):

if res == "" or end-start+1 < len(res):

res = s[start:end+1]

left_char = s[start]

if left_char in t:

if char_frequency[left_char] == 0:

chars_matched -= 1

char_frequency[left_char] += 1

start += 1

return res