删了键之后呢

同样的滑动，这次减到 0 就把键删掉。运行它：

def add(cnt, ch):
    cnt[ch] = cnt.get(ch, 0) + 1

def remove(cnt, ch):
    cnt[ch] -= 1
    if cnt[ch] == 0:
        del cnt[ch]

cnt = {}
s = "abcabcbb"
k = 3
for i in range(k):
    add(cnt, s[i])
for i in range(k, len(s)):
    add(cnt, s[i])
    remove(cnt, s[i - k])
print(len(cnt))