Four Divisors

Description

Given an integer array nums, return the sum of divisors of the integers in that array that have exactly four divisors.

If there is no such integer in the array, return 0.

 

Example 1:

Input: nums = [21,4,7]
Output: 32
Explanation:
21 has 4 divisors: 1, 3, 7, 21
4 has 3 divisors: 1, 2, 4
7 has 2 divisors: 1, 7
The answer is the sum of divisors of 21 only.

 

Constraints:

• 1 <= nums.length <= 10^4
• 1 <= nums[i] <= 10^5

Solution(javascript)

/** https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
 * @param {number[]} nums
 * @return {number}
 */
const max = 10 ** 5
const memo = new Array(max + 1).fill(0).map(() => [1])
for (let divisor = 2; divisor <= max; divisor++) {
  for (let num = 1; num * divisor <= max; num++) {
    memo[num * divisor].push(divisor)
  }
}
const sumFourDivisors = function (nums) {
  const sum = (arr = []) => arr.reduce((acc, x) => acc + x, 0)
  return nums.reduce((acc, num) => {
    if (memo[num].length === 4) {
      return acc + sum(memo[num])
    }
    return acc
  }, 0)
}