1819. Number of Different Subsequences GCDs
Description
You are given an array nums
that consists of positive integers.
The GCD of a sequence of numbers is defined as the greatest integer that divides all the numbers in the sequence evenly.
- For example, the GCD of the sequence
[4,6,16]
is2
.
A subsequence of an array is a sequence that can be formed by removing some elements (possibly none) of the array.
- For example,
[2,5,10]
is a subsequence of[1,2,1,2,4,1,5,10]
.
Return the number of different GCDs among all non-empty subsequences of nums
.
Example 1:
![](https://spcdn.pages.dev/leetcode/problems/1819.Number%20of%20Different%20Subsequences%20GCDs/images/image-1.png)
Input: nums = [6,10,3] Output: 5 Explanation: The figure shows all the non-empty subsequences and their GCDs. The different GCDs are 6, 10, 3, 2, and 1.
Example 2:
Input: nums = [5,15,40,5,6] Output: 7
Constraints:
1 <= nums.length <= 105
1 <= nums[i] <= 2 * 105
Solutions
Solution 1: Enumeration + Mathematics
For all sub-sequences of the array $nums$, their greatest common divisor (GCD) will not exceed the maximum value $mx$ in the array.
Therefore, we can enumerate each number $x$ in $[1,.. mx]$, and determine whether $x$ is the GCD of a sub-sequence of the array $nums$. If it is, then we increment the answer by one.
So the problem is transformed into: determining whether $x$ is the GCD of a sub-sequence of the array $nums$. We can do this by enumerating the multiples $y$ of $x$, and checking whether $y$ exists in the array $nums$. If $y$ exists in the array $nums$, then we calculate the GCD $g$ of $y$. If $g = x$ occurs, then $x$ is the GCD of a sub-sequence of the array $nums$.
The time complexity is $O(n + M \times \log M)$, and the space complexity is $O(M)$. Here, $n$ and $M$ are the length of the array $nums$ and the maximum value in the array $nums$, respectively.
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