How should I practice Count Ordered Combination Sums with Negative Values?

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Problem · Array

Count Ordered Combination Sums with Negative Values

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● Hard

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Problem statement

Given an array of distinct nonzero integers candidates, an integer target, and a positive integer maxLength, return the number of nonempty ordered sequences of at most maxLength values whose sum is target.

You may reuse a candidate any number of times.
Two sequences with the same values in different orders count separately.
Sequences of different lengths count separately.

Function

countOrderedCombinationSumsWithNegatives(candidates: int[], target: int, maxLength: int) → int

Examples

Example 1

candidates = [1,-1]target = 0maxLength = 2return = 2

The valid nonempty sequences are [1,-1] and [-1,1]. The empty sequence is not counted.

Example 2

candidates = [2,-1]target = 3maxLength = 3return = 3

The valid sequences are the three orderings of [2,2,-1].

Example 3

candidates = [-2,-1]target = -3maxLength = 3return = 3

The valid sequences are [-2,-1], [-1,-2], and [-1,-1,-1].

Constraints

1 <= candidates.length <= 30.
-100 <= candidates[i] <= 100 and candidates[i] != 0.
All values in candidates are distinct.
-1000 <= target <= 1000.
1 <= maxLength <= 20.
The correct answer fits a signed 32-bit integer.