560 Subarray Sum Equals K

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

Example 1:

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

Output:
 2

Note:

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

Solution)

Brute-Force

class Solution {
    public int subarraySum(int[] nums, int k) {
        if (nums == null || nums.length == 0) return 0;
        int count = 0;
        for (int i = 0; i < nums.length; i++) {
            int curSum = 0;
            for (int j = i; j < nums.length; j++) {
                curSum += nums[j];
                if (curSum == k) count++;
            }
        }

        return count;
    }
}

Optimization by using a map

class Solution {
    public int subarraySum(int[] nums, int k) {
        int ans = 0, sum = 0;
        Map<Integer, Integer> map = new HashMap<>();
        for (int num : nums) {
            map.put(-sum, map.getOrDefault(-sum, 0) + 1);
            sum += num;
            ans += map.getOrDefault(k - sum, 0);
        }
        return ans;
    }
}