leet minimum-window-substring

# https://leetcode.com/problems/minimum-window-substring/
from collections import defaultdict, Counter


class Solution:
    def minWindow(self, s: str, t: str) -> str:
        res = ''
        if not s or not t:
            return res

        d = dict(Counter(t))
        n = len(d)

        d_cur = defaultdict(int)
        idx_good = defaultdict(bool)
        idx_good_count = 0

        l, r = 0, -1
        while True:
            if idx_good_count < n:
                r += 1
                if r > len(s) - 1:
                    break

                ch = s[r]
                if ch in d:
                    d_cur[ch] += 1
                    if d_cur[ch] >= d[ch] and not idx_good[ch]:
                        idx_good[ch] = True
                        idx_good_count += 1
            else:
                l += 1
                prev_ch = s[l - 1]
                if prev_ch in d:
                    d_cur[prev_ch] -= 1
                    if d_cur[prev_ch] < d[prev_ch] and idx_good[prev_ch]:
                        idx_good[prev_ch] = False
                        idx_good_count -= 1

            if idx_good_count == n:
                if (res and len(s[l:r + 1]) < len(res)) or not res:
                    res = s[l:r + 1]

        return res