LeetCode #523: Continuous Subarray Sum (C/C++).

medium

source: https://leetcode.com/problems/continuous-subarray-sum/
C/C++ Solution to LeetCode problem 523. Continuous Subarray Sum.

Problem

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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.

Examples

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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

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

Solution

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class Solution {
public:
  bool checkSubarraySum(vector<int>& nums, int k) {
    int sum = 0;
    unordered_map<int, int> hashmods{ {0, 0} };
    for (int i = 0; i < nums.size(); i+= 1) {
      sum += nums[i];
      int mod = sum % k;
      if (!hashmods.count(mod))
        hashmods[mod] = i + 1;
      else if (hashmods[mod] < i)
        return true;
    }

    return false;
  }
};