523. Continuous Subarray Sum

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523. Continuous Subarray Sum

Description

Difficulty: Medium

Related Topics: Array, Hash Table, Math, Prefix Sum

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:

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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:

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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:

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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

Hints/Notes

Solution

Language: C++

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class Solution {
public:
    bool checkSubarraySum(vector<int>& nums, int k) {
        int size = nums.size();
        vector<int> preSum(size + 1, 0);
        for (int i = 0; i < size; i++) {
            preSum[i + 1] = preSum[i] + nums[i];
        }
        map<int, int> m;
        for (int i = 0; i <= size; i++) {
            if (m.count(preSum[i] % k)) {
                int idx = m[preSum[i] % k];
                if (i - idx > 1) {
                    return true;
                }
            } else {
                m[preSum[i] % k] = i;
            }
        }
        return false;
    }
};