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523. Continuous Subarray Sum 🔗

Date: Jun 09, 2024

Tags: prefix-sum

Description

Given an integer array nums and an integer k, return true if nums has a good subarray or false otherwise.

A good subarray is a subarray where:

  • its length is at least two, and
  • the sum of the elements of the subarray is a multiple of k.

Note that:

  • A subarray is a contiguous part of the array.
  • 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.

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
      

Constraints:

  • 1 <= nums.length <= 105
  • 0 <= nums[i] <= 109
  • 0 <= sum(nums[i]) <= 231 - 1
  • 1 <= k <= 231 - 1

Optimal

function checkSubarraySum(nums: number[], k: number): boolean {
  const map = new Set();
  let sum = 0;

  for (const x of nums) {
    const prev = sum;

    sum = (sum + x) % k;

    if (map.has(sum)) return true;

    map.add(prev);
  }

  return false;
}

export { checkSubarraySum };

Brute-force

function checkSubarraySum(nums: number[], k: number): boolean {
  for (let i = 0; i < nums.length; i++) {
    let current = 0;
    for (let j = i; j < nums.length; j++) {
      current += nums[j];
      if (j > i && current % k === 0) return true;
    }
  }

  return false;
}

export { checkSubarraySum };