2183. Count Array Pairs Divisible by K

题目

Given a 0-indexed integer array nums of length n and an integer k, return the number of pairs (i, j) such that:

0 <= i < j <= n - 1 and
nums[i] * nums[j] is divisible by k.

Example 1:

Input: nums = [1,2,3,4,5], k = 2
Output: 7
Explanation:
The 7 pairs of indices whose corresponding products are divisible by 2 are
(0, 1), (0, 3), (1, 2), (1, 3), (1, 4), (2, 3), and (3, 4).
Their products are 2, 4, 6, 8, 10, 12, and 20 respectively.
Other pairs such as (0, 2) and (2, 4) have products 3 and 15 respectively, which are not divisible by 2.

Example 2:

Input: nums = [1,2,3,4], k = 5
Output: 0
Explanation: There does not exist any pair of indices whose corresponding product is divisible by 5.

Constraints:

1 <= nums.length <= 10^5
1 <= nums[i], k <= 10^5

题目大意

给你一个下标从 0 开始、长度为 n 的整数数组 nums 和一个整数 k ，返回满足下述条件的下标对 (i, j) 的数目：

0 <= i < j <= n - 1 且
nums[i] * nums[j] 能被 k 整除。

解题思路

先找出 num 中每个元素与 k 的最大公约数。并统计这些公约数出现的频次，将数据保存在 map 中。在计算过程中，循环可以只需算到 ${O(\sqrt {k})}$ , 因为每一个 gcd[i] 一定是 k 的因数，而它出现的频次不会超过 ${O(\sqrt {k})}$。简单证明一下：假设因子 v 和 k/v 这两个因数为 k 的因子。v 和 k/v 必至少有 1 个小于等于 $\sqrt {k}$。所以 k 的因子也不会超过 2 * $\sqrt {k}$ = ${O(\sqrt {k})}$ 个。
算出上述的 map 以后，2 层循环暴力遍历 key 值，如果 a * b 能被 k 整除，并且 a 和 b 不相同，那么 a 和 b 对应的 value 值相乘即为满足条件的下标对数；如果 a 和 b 相同，那么下标对数为 $C_{n}^{2}$。最后累加结果即可。

代码

package leetcode

import "math"

func countPairs(nums []int, k int) int64 {
	n := int(math.Sqrt(float64(k)))
	gcds, res := make(map[int]int, n), 0
	for _, num := range nums {
		gcds[gcd(num, k)]++
	}

	for a, n1 := range gcds {
		for b, n2 := range gcds {
			if a > b || (a*b)%k != 0 {
				continue
			}
			if a != b {
				res += n1 * n2
			} else { // a == b
				res += n1 * (n1 - 1) / 2
			}
		}
	}
	return int64(res)
}

func gcd(a, b int) int {
	for a%b != 0 {
		a, b = b, a%b
	}
	return b
}

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