MaxDoubleSliceSum

🔗 質問リンク
https://app.codility.com/programmers/lessons/9-maximum_slice_problem/max_double_slice_sum/start/
問題の説明
A non-empty array A consisting of N integers is given.
A triplet (X, Y, Z), such that 0 ≤ X < Y < Z < N, is called a double slice.
The sum of double slice (X, Y, Z) is the total of A[X + 1] + A[X + 2] + ... + A[Y − 1] + A[Y + 1] + A[Y + 2] + ... + A[Z − 1].
For example, array A such that:

A[0] = 3
A[1] = 2
A[2] = 6
A[3] = -1
A[4] = 4
A[5] = 5
A[6] = -1
A[7] = 2

contains the following example double slices:
double slice (0, 3, 6), sum is 2 + 6 + 4 + 5 = 17,
double slice (0, 3, 7), sum is 2 + 6 + 4 + 5 − 1 = 16,
double slice (3, 4, 5), sum is 0.
The goal is to find the maximal sum of any double slice.
Write a function:
def solution(A)
that, given a non-empty array A consisting of N integers, returns the maximal sum of any double slice.
For example, given:

A[0] = 3
A[1] = 2
A[2] = 6
A[3] = -1
A[4] = 4
A[5] = 5
A[6] = -1
A[7] = 2

the function should return 17, because no double slice of array A has a sum of greater than 17.
⚠▼制限

N is an integer within the range [3..100,000];

each element of array A is an integer within the range [−10,000..10,000].

💡 プール(言語:Python)
これは難題です.人の解答を見て、書き直した.解の考え方は,Yを基準として左と右の数の和を分けて計算し,2つの配列を形成し,Yを基準として左と右の数の和の最大値を求めることである.この考えを見て、理解にも時間がかかりました.あと全部負数にならないケースってありますよね?ただし条件はX,Y,Zが0以上とする.また今度よく読みましょう.時間複雑度O(N)O(N)O(N)O(N)

def solution(A):
    length = len(A)
    leftSum = [0 for i in range(length)]
    rightSum = [0 for i in range(length)]
    maximum = 0

    for i in range(1, length-1):
        leftSum[i] = max(0, leftSum[i-1] + A[i])
    
    for i in range(length-2, 0, -1):
        rightSum[i] = max(0, rightSum[i+1] + A[i])

    for i in range(1, length-1):
        maximum = max(maximum, leftSum[i-1] + rightSum[i+1])
    
    return maximum