Greater Than

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题目

In This Challenge, You should implement a type GreaterThan<T, U> like T > U

Negative numbers do not need to be considered.

For example

GreaterThan<2, 1>; //should be true
GreaterThan<1, 1>; //should be false
GreaterThan<10, 100>; //should be false
GreaterThan<111, 11>; //should be true

Good Luck!

解答

数字比大小，在 ts 中可以通过构建数组的方式，

将数字构建为数组，例如：

type NumberToArray<T extends number, U extends any[] = []> = [T] extends [
  U["length"]
]
  ? U
  : NumberToArray<T, [...U, ""]>;

// type A = ["", "", ""]
type A = NumberToArray<3>;

// type B = [""]
type B = NumberToArray<1>;

只要 B 无法满足 [...A, ...infer _]，那么 B 就比 A 的长度要小，

继而能判断出 B 小于 A：

type R = B extends [...A, ...infer _] ? false : true;

最后得出答案：

type NumberToArray<T extends number, U extends any[] = []> = [T] extends [
  U["length"]
]
  ? U
  : NumberToArray<T, [...U, ""]>;

type GreaterThan<
  T extends number,
  U extends number
> = NumberToArray<U> extends [...NumberToArray<T>, ...infer _] ? false : true;

上述答案无法判断 bigint，可以浏览精选看看大家对于 bigint 的讨论。

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I improved your answer as follows:

// compares the first digits of T and U. If they are the same - compare rest of T and rest of U
// prettier-ignore
type GreaterThanSameDigitCount<
  T extends number | string,
  U extends number | string
> = `${T}` extends `${infer TF}${infer TR}`
  ? `${U}` extends `${infer UF}${infer UR}`
    ? TF extends UF
      ? GreaterThanSameDigitCount<TR, UR>
      : "0123456789" extends `${string}${TF}${string}${UF}${string}`
        ? false
        : true
    : true
  : false;

type DigitsToArr<S extends string> = S extends `${string}${infer R}`
  ? [0, ...DigitsToArr<R>]
  : [];

type ArrLenCompare<
  T extends any[],
  U extends any[]
> = "0123456789" extends `${string}${T["length"]}${string}${U["length"]}${string}`
  ? -1
  : "0123456789" extends `${string}${U["length"]}${string}${T["length"]}${string}`
  ? 1
  : 0;

type GreaterThan<T extends number, U extends number> = ArrLenCompare<
  DigitsToArr<`${T}`>,
  DigitsToArr<`${U}`>
> extends 0
  ? GreaterThanSameDigitCount<T, U>
  : ArrLenCompare<DigitsToArr<`${T}`>, DigitsToArr<`${U}`>> extends 1
  ? true
  : false;