523. Continuous Subarray Sum

Medium

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

Solution:

class Solution {

    /**
     * @param Integer[] $nums
     * @param Integer $k
     * @return Boolean
     */
    function checkSubarraySum($nums, $k) {
        $prefixMod = 0;
        $modSeen = array();
        $modSeen[0] = -1;
        for ($i = 0; $i < count($nums); $i++) {
            $prefixMod = ($prefixMod + $nums[$i]) % $k;
            if (array_key_exists($prefixMod, $modSeen)) {
                if ($i - $modSeen[$prefixMod] > 1) return 1;
            } else {
                $modSeen[$prefixMod] = $i;
            }
        }
        return 0;
    }
}

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