Day 22 of 30 days Leetcode challenge - Subarray Sum Equals K

Subarray Sum Equals K

Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.

Example 1:

Input:nums = [1,1,1], k = 2
Output: 2

Note:

• The length of the array is in range [1, 20,000].
• The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

Solution

Prefix sum with hash table
sum(i,j) = sum(0, j) - sum(0,i)
O(n) time and O(n) space

class Solution {
    func subarraySum(_ nums: [Int], _ k: Int) -> Int {
        var cSum = 0
        var previousSum = [0: 1]
        var result = 0
        
        for val in nums {
            cSum += val
            
            if let found = previousSum[cSum - k] {
                result += found
            }
            
            previousSum[cSum] = (previousSum[cSum] ?? 0) + 1
        }
        return result
    }
}