Continuous Subarray Sum

Published: Oct 26, 2022

Medium Prefix Sum Hash Table Array

Introduction

This problem is very similar to Subarray Sum Equals K. Instead of “sum equals k”, it is “sum divisible by k” here. However, we can solve this problem in the same way. Using an accumulated sum and hash table, go through like a 2-sum. The main differences are the hash table keys and values. Also, how to identify the answer.

Problem Description

Given an integer array nums and an integer k, return true if nums has a continuous subarray of size at least two whose elements sum up to a multiple of k, or false otherwise.

An integer x is a multiple of k if there exists an integer n such that x = n * k. 0 is always a multiple of k.

Constraints:

  • 1 <= nums.length <= 10**5
  • 0 <= nums[i] <= 10**9
  • 0 <= sum(nums[i]) <= 2**31 - 1
  • 1 <= k <= 2**31 - 1

https://leetcode.com/problems/continuous-subarray-sum/

Examples

Example 1
Input: nums = [23,2,4,6,7], k = 6
Output: true
Explanation: [2, 4] is a continuous subarray of size 2 whose elements sum up to 6.
Example 2
Input: nums = [23,2,6,4,7], k = 6
Output: true
Explanation: [23, 2, 6, 4, 7] is an continuous subarray of size 5 whose elements sum up to 42.
42 is a multiple of 6 because 42 = 7 * 6 and 7 is an integer.
Example 3
Input: nums = [23,2,6,4,7], k = 13
Output: false

Analysis

The solution starts from initializing the accumulated sum and hash table. The hash table’s key is the accumulated sum modulo k. The value is an index. Given that, the hash table’s initial values is {0: -1}. When going over the given array, calculate the accumulated sum. If the accumulated sum modulo k is not in the hash table, save it along with the index. Otherwise, check the index difference. If it satisfies the condition, return True.

Solution

class ContinuousSubarraySum:
    def checkSubarraySum(self, nums: List[int], k: int) -> bool:
        acc, memo = 0, {0: -1}
        for idx, v in enumerate(nums):
            acc += v
            if acc % k not in memo:
                memo[acc % k] = idx
            elif idx - memo[acc % k] >= 2:
                return True
        return False

Complexities

  • Time: O(n)
  • Space: O(n)