打平数组

// 考虑递归数组
function myFlat(arr: Array<unknown>): unknown[] {
  const cache = new WeakMap<any, any>()
  function _myFlat(arr: Array<unknown>): unknown[] {
    if (!Array.isArray(arr)) return [arr]
    if (cache.has(arr)) {
      throw new Error('递归数组!')
    }

    const result: unknown[] = []
    cache.set(arr, result)
    for (const a of arr) {
      if (Array.isArray(a)) {
        const temp = _myFlat(a)
        result.push(...temp)
      } else {
        result.push(a)
      }
    }

    return result
  }

  return _myFlat(arr)
}

function easyFlat(arr: Array<unknown>): unknown[] {
    return arr.reduce((prev: unknown[], cur:unknown) => Array.isArray(cur) ? [...prev, ...easyFlat(cur)] : [...prev, cur], [])
}