Given a number N, find the Nth triangular number, i.e. 1 + 2 + 3 + ... + N
/* _____________ Your Code Here _____________ */
export type NumberToArray<
T extends number,
R extends 1[] = []
> = R["length"] extends T ? R : NumberToArray<T, [...R, 1]>;
type Triangular<N extends number, R extends any[] = [], Count extends unknown[] = []> = Count['length'] extends N
? R['length']
: Triangular<N, [...NumberToArray<[...Count, ""]['length']>, ...R], [...Count, unknown]>
/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'
type cases = [
Expect<Equal<Triangular<0>, 0>>,
Expect<Equal<Triangular<1>, 1>>,
Expect<Equal<Triangular<3>, 6>>,
Expect<Equal<Triangular<10>, 55>>,
Expect<Equal<Triangular<20>, 210>>,
Expect<Equal<Triangular<55>, 1540>>,
Expect<Equal<Triangular<100>, 5050>>,
]
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