Sliding Window

"""
Pseudocode:

# sliding window - need the full object to slide over and the size of the window
    # handle the edge case where the window is bigger than the array
    # calculate the initial window and set the best-found-value to it
    # create a pointer to the start of the window
    # loop through valid values for the end of the window
        # calculate window value
        # update best answer found so far
        # increment start of window
    # return the best answer you found

Mistakes I've made:
- I forgot to increment the 'start' index variable.
- I initialized l to k-1 instead of 0. I guess I got it confused with the r pointer.
"""

def max_subarray(arr, k):
    if k > len(arr):
        return -1
    max_sum = window_sum = sum(arr[:k])
    l = 0
    for r in range(k, len(arr)):
        window_sum = window_sum - arr[l] + arr[r]
        max_sum = max(window_sum, max_sum)
        l += 1
    return max_sum

arr = [2, 3, 4, 1, 5]
print(max_subarray(arr, 3))  # Output: 10, which is the sum of [4, 1, 5]