滑动窗口与前缀和

for right in range(n):
  add(a[right])
  if right - left + 1 > k: remove(a[left]); left += 1
  if right - left + 1 == k: update()

for right in range(n):
  add(right)
  while need_shrink():
    update_if_minimum()
    remove(left); left += 1
  update_if_maximum()

count = {0: 1}
prefix = answer = 0
for value in a:
  prefix += value
  answer += count.get(prefix - k, 0)
  count[prefix] += 1