8804 - Two Sum

[alexfung888]

type Encode<X extends number, R extends unknown[] = []> = R['length'] extends X ? R : Code<X, [...R, unknown]>;

type Add<X extends number, Y> = Y extends any ? [...Encode<X>, ...Encode<Y>]['length'] : never;
// assumes X is a single number, Y can be unions like 3|4|5

type PickAdd<T extends number[]> = T extends [infer Head extends number, ...infer Tail extends number[]]
   ? Add<Head, Tail[number]> | PickAdd<Tail>
   : never;
// pick a table entry and ADD<> to the other entries of the array

type TwoSum<T extends number[], U extends number> = U extends PickAdd<T> ? true : false;
// convert result to required boolean