Triangular number

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题目

Given a number N, find the Nth triangular number, i.e. 1 + 2 + 3 + ... + N

解答

计算的题目基本上都用数组来实现，利用数组的 length 求和，这题相当于需要把每个数转成对应长度的数组，例如 3，需要转成 1，2，3 这三个数字对应长度的数组

1: ['']
2: ['', ''],
3: ['', '', ''],

递归创建即可

type CountArr<N extends number, R extends string[] = []> = R["length"] extends N
  ? R
  : CountArr<N, [...R, ""]>;

type Triangular<
  N extends number,
  R extends any[] = [],
  Count extends string[] = []
> = Count["length"] extends N
  ? R["length"]
  : Triangular<
      N,
      [...CountArr<[...Count, ""]["length"]>, ...R],
      [...Count, ""]
    >;