Minimum Window Substring

class Solution:
    def minWindow(self, s: str, t: str) -> str:
        if not s or not t:
            return ""
        need = {}
        for c in t:
            need[c] = need.get(c, 0) + 1
        required = len(need)
        have = 0
        window = {}
        best = [-1, -1]
        best_len = float("inf")
        left = 0
        for right in range(len(s)):
            c = s[right]
            window[c] = window.get(c, 0) + 1
            if c in need and window[c] == need[c]:
                have += 1
            while have == required:
                if (right - left + 1) < best_len:
                    best = [left, right]
                    best_len = right - left + 1
                window[s[left]] -= 1
                if s[left] in need and window[s[left]] < need[s[left]]:
                    have -= 1
                left += 1
        l, r = best
        return s[l : r + 1] if best_len != float("inf") else ""