Subsequence

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题目

Given an array of unique elements, return all possible subsequences.

A subsequence is a sequence that can be derived from an array by deleting some or no elements without changing the order of the remaining elements.

For example:

type A = Subsequence<[1, 2]>; // [] | [1] | [2] | [1, 2]

解答

[a,...[b] | [c]] => [a,b] | [a,c]

[...Subsequence<Rest>] | [One, ...Subsequence<Rest>]
[1] => [] | [1,...[]] => [] | [1]
[1,2] => [] | [2] | [[] | [1] ,...[] | [2]] => [] | [2] | [1] | [1,2]
[1,2,3] => [] | [3] | [[] | [2] | [1] | [1,2],...[] | [3]] => [] | [3] | [2] | [1] | [1,2] | [2,3] | [1,3] | [1,2,3]

type Subsequence<T> = T extends [infer One, ...infer Rest]
  ? [One] | [...Subsequence<Rest>] | [One, ...Subsequence<Rest>]
  : [];