<code python>
class Solution:
    def minWindow(self, s: str, t: str) -> str:
        i, j, n, m = 0, 0, len(s), len(t)
        tot, ans = 0, (1e9, (0, -1))
        chs = {c:t.count(c) for c in t}
        for j in range(n):
            if s[j] in chs:
                chs[s[j]] -= 1
                if chs[s[j]] >= 0:
                    tot += 1
                while tot == m:
                    if s[i] in chs:
                        chs[s[i]] += 1
                        if chs[s[i]] > 0:
                            tot -= 1
                    if ans[0] > j-i+1:
                        ans = j-i+1, (i, j)
                    i += 1
        return s[ans[1][0]:ans[1][1]+1]
</code>