R7-子串篇

class Solution:
    def minWindow(self, s: str, t: str) -> str:
        ret_left,ret_right=-1,len(s)
        left=0
        #s子串字母出现的次数
        cnt_s=Counter()
        #t中字母出现的次数
        cnt_t=Counter(t)
        for right,c in enumerate(s):
            cnt_s[c]+=1
            while cnt_s>=cnt_t:
                #找到更短的子串
                if right-left<ret_right-ret_left:
                    ret_left,ret_right=left,right
                cnt_s[s[left]]-=1
                left+=1
        return ""if ret_left<0 else s[ret_left:ret_right+1]