数组与双指针

slow = 0
for fast in range(n):
  if should_keep(a[fast]):
    a[slow] = a[fast]
    slow += 1

left, right = 0, n - 1
while left < right:
  update_answer(left, right)
  if move_left_condition: left += 1
  else: right -= 1

sort(a)
for i in range(n):
  if duplicate_i: continue
  left, right = i + 1, n - 1
  while left < right: compare_sum()