# 523. Continuous Subarray 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

## Solutions

• class Solution {
public boolean checkSubarraySum(int[] nums, int k) {
Map<Integer, Integer> mp = new HashMap<>();
mp.put(0, -1);
int s = 0;
for (int i = 0; i < nums.length; ++i) {
s += nums[i];
int r = s % k;
if (mp.containsKey(r) && i - mp.get(r) >= 2) {
return true;
}
if (!mp.containsKey(r)) {
mp.put(r, i);
}
}
return false;
}
}

• class Solution {
public:
bool checkSubarraySum(vector<int>& nums, int k) {
unordered_map<int, int> mp;
mp[0] = -1;
int s = 0;
for (int i = 0; i < nums.size(); ++i) {
s += nums[i];
int r = s % k;
if (mp.count(r) && i - mp[r] >= 2) return true;
if (!mp.count(r)) mp[r] = i;
}
return false;
}
};

• class Solution:
def checkSubarraySum(self, nums: List[int], k: int) -> bool:
s = 0
mp = {0: -1}
for i, v in enumerate(nums):
s += v
r = s % k
if r in mp and i - mp[r] >= 2:
return True
if r not in mp:
mp[r] = i
return False


• func checkSubarraySum(nums []int, k int) bool {
mp := map[int]int{0: -1}
s := 0
for i, v := range nums {
s += v
r := s % k
if j, ok := mp[r]; ok && i-j >= 2 {
return true
}
if _, ok := mp[r]; !ok {
mp[r] = i
}
}
return false
}