Re: [閒聊] 每日leetcode

[sustainer123 (caster )]

https://leetcode.com/problems/subarray-sums-divisible-by-k

974. Subarray Sums Divisible by K

給定一數列nums與整數k

請回傳能被k整除的子序列和之非空子序列數量

此子序列必須是nums的連續部分

Example 1:

Input: nums = [4,5,0,-2,-3,1], k = 5
Output: 7
Explanation: There are 7 subarrays with a sum divisible by k = 5:
[4, 5, 0, -2, -3, 1], [5], [5, 0], [5, 0, -2, -3], [0], [0, -2, -3], [-2, -3]
Example 2:

Input: nums = [5], k = 9
Output: 0


Constraints:

1 <= nums.length <= 3 * 104
-104 <= nums[i] <= 104
2 <= k <= 104

思路:
這題就昨天那題的變形

昨天用集合紀錄前綴和

今天需要知道子序列數量 所以要改用哈希表

哈希表紀錄該前綴和出現幾次

把它加到result再回傳就結束了




Python Code:
class Solution:
    def subarraysDivByK(self, nums: List[int], k: int) -> int:
        nums.insert(0,0)
        prefix = defaultdict(int)
        prefix[0] = 1
        result = 0


        for i in range(1,len(nums)):
            nums[i] = (nums[i]+nums[i-1])%k
            result += prefix[nums[i]]
            prefix[nums[i]] += 1

        return result



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