continuousSubarraySum.swift

/*
523. Continuous Subarray Sum

Given a list of non-negative numbers and a target integer k, write a function to check if the array has a continuous subarray of size at least 2 that sums up to a multiple of k, that is, sums up to n*k where n is also an integer.

 

Example 1:

Input: [23, 2, 4, 6, 7],  k=6
Output: True
Explanation: Because [2, 4] is a continuous subarray of size 2 and sums up to 6.

Example 2:

Input: [23, 2, 6, 4, 7],  k=6
Output: True
Explanation: Because [23, 2, 6, 4, 7] is an continuous subarray of size 5 and sums up to 42.

 

Note:

    1.The length of the array won't exceed 10,000.
    2.You may assume the sum of all the numbers is in the range of a signed 32-bit integer.

https://leetcode.com/problems/continuous-subarray-sum/
*/
class Solution {
    func checkSubarraySum(_ nums: [Int], _ k: Int) -> Bool {
        /*
        corner case: if the very first subarray with first two numbers in array could form the result, 
        we need to put mod value 0 and index -1 to make it as a true case
        */
        var dict = [0: -1]
        var sum = 0
        
        for i in 0..<nums.count {
            sum += nums[i]
            if k != 0 {
                sum %= k
            }
            if let index = dict[sum] {
                if i - index > 1 {
                    return true
                }
            } else {
                dict[sum] = i
            }
        }
        
        return false
    }
}