滑动窗口

def subarr_max_sum(arr, k):
  n = len(arr)
  if n <= k:
    return -1
  window_sum = 0
  max_sum = window_sum
  # cache current window result
  window_sum = sum(arr[0:k])
  for i in range(n-k):
    window_sum = window_sum - arr[i] + arr[i+k]
    max_sum = max(window_sum, max_sum)
  return max_sum
# Driver code
arr = [1, 4, 2, 10, 2, 3, 1, 0, 20]
k = 4
print(subarr_max_sum(arr, k))

def subarr_min_len(arr, x):
  n = len(arr)
  i, j = 0, 0
  sum = 0
  min_len = float('inf')
  while j < n:
    # move right pointer to increase the sum, 
    # when the sum is greater than x, the elements
    # after current j are ignored. 
    while j < n and sum <= x:
      sum += arr[j]
      j = j + 1

    # when we reach the end of the array,
    # and the sum is smaller than x,
    # the rest of cases are ignored.
    if j == n and sum  <= x:
      break

    # move left pointer to decrease the sum, 
    # when the sum is smaller than x, the element 
    # before current i are ignored. 
    while i < j and sum - arr[i] > x:
      sum -= arr[i]
      i = i + 1
    
    min_len = min(min_len, j - i)

    # move left pointer
    sum -= arr[i]
    i += 1
  if  min_len == float('inf'):
    return 0
  return min_len
# Driver code
arr = [1, 4, 45, 6, 10, 19]
x = 51
print(subarr_min_len(arr, x))