最小覆盖子串：
给出两个字符串 s 和 t，要求在 s 中找出最短的包含 t 中所有字符的连续子串。
class Solution:
def minWindow(self , S , T ):
lens=len(S)
needcnt={}
for i in T:
needcnt[i]=1 if i not in needcnt else needcnt[i]+1
totalneed=len(needcnt)
minlen=len(S)+1
minval=""
i=0
lasti=0
while i<lens:
if totalneed==0:
while totalneed==0:
if minlen>=i-lasti:
minlen=i-lasti
minval=S[lasti:i]
if S[lasti] in needcnt:
needcnt[S[lasti]]=needcnt[S[lasti]]+1
if needcnt[S[lasti]]==1:
totalneed=totalneed+1
lasti=lasti+1
else:
if S[i] in needcnt:
needcnt[S[i]]=needcnt[S[i]]-1
if needcnt[S[i]]==0:
totalneed=totalneed-1
i=i+1
if i==lens:
if totalneed==0:
while totalneed==0:
if minlen>=i-lasti:
minlen=i-lasti
minval=S[lasti:i]
if S[lasti] in needcnt:
needcnt[S[lasti]]=needcnt[S[lasti]]+1
if needcnt[S[lasti]]==1:
totalneed=totalneed+1
lasti=lasti+1
return minval
