76. Minimum Window Substring

class Solution:
    def minWindow(self, s: str, t: str) -> str:
        # time O(S+T) space O(T)
        # step1: initialize l, minL, minLen, cnt_s, cnt_t. where cnt_s record valid ch in s for t. 
        # step2: record freq for each char in t
        d = {}
        for c in t:
            d[c] = d.get(c,0)+1
        l = minL = 0
        size_s =  len(s)
        min_len = len(s) +1 
        cnt_t = len(t)
        cnt_s = 0 
        # step3: loop through s using r as pointer, if found s[r] in d, update d and increment cnt_s if s[r] in d >= 0
        for r in range(size_s):
            if s[r] in d:
                d[s[r]] -= 1
                if d[s[r]] >= 0:
                    cnt_s += 1 
            # step3.1: when cnt_s == cnt_t, update minL and min_len 
            while cnt_s == cnt_t:
                if r-l+1 < min_len:
                    minL = l 
                    min_len = r- l + 1 
                # step3.2: check if s[l] in d, if yes, update d, check if s[l] valid in t by checking if d[s[l]]> 0, note not >= 0 .   decrement cnt_s 
                if s[l] in d:
                    d[s[l]] += 1
                    if d[s[l]] > 0:
                        cnt_s -= 1
                # step3.3 left ptr move ++ 
                l += 1 
        # step4: check if min_len > len(s)
        if min_len > len(s):
            return ""
        else:
            return s[minL:minL+min_len]