003. max subarray sum(lambda and map func)

# accumulate culculate method
def max_subarray_sum(arr):
    max_sum = arr[0]
    current_sum = arr[0]
    
    for i in arr[1:]:## start from the second element
        current_sum = max(i, current_sum + i)## as long as the current_sum > 0, i + current_sum will large than i. So we can let the subarray accumulate until the sum is maximum
        max_sum = max(current_sum, max_sum) ## after recording every turn of current_max, we will know which subarray is the largest one.
    return max_sum

arr = [1, -2, 3, 4, -1, 2, 1, -5, 4]
result = max_subarray_sum(arr)
print("max subarray sum is:", result)

# My algorithm-use map function:
# The map function is used to convert each str generated by record.split() into int
# The map function is very convenient because it allows you to operate on the elements in an iterable object and generate a new resulting list without using an explicit loop.
# The map function is a very useful Python built-in function that allows you to apply a function to each element of an iterable object (such as a list, tuple, etc.), and then return a new iterable object containing The result of function processing.
def subarray():
    record = input()
    record_list = list(map(int, record.split()))
    record_sum = int(record_list[0])
    record_total = int(record_list[0])
    for i in record_list[1:]:
        record_sum = max(i, record_sum + i)
        record_total = max(record_sum, record_total)
    return record_total

# My force resolution:
def max_subarray_sum_bruteforce(arr):
     max_sum = float('-inf')

     for i in range(len(arr)):
         current_sum = 0
         for j in range(i, len(arr)):
             current_sum += arr[j]
             max_sum = max(max_sum, current_sum)

     return max_sum

arr = [1, -2, 3, 4, -1, 2, 1, -5, 4]
result = max_subarray_sum_bruteforce(arr)
print("Maximum subarray sum (Brute Force):", result)

subarray()