560. Subarray Sum Equals K

// LeetCode URL: https://leetcode.com/problems/subarray-sum-equals-k/
import java.util.HashMap;

/**
 * Maintain a count of cumulative sum in a map.
 *
 * Refer: https://leetcode.com/problems/subarray-sum-equals-k/discuss/190674
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(N)
 *
 * N = Length of the input nums array.
 */
class Solution {
    public int subarraySum(int[] nums, int k) {
        if (nums == null || nums.length == 0) {
            return 0;
        }

        int count = 0;
        int sum = 0;
        HashMap<Integer, Integer> map = new HashMap<>();
        map.put(0, 1);

        for (int num : nums) {
            sum += num;
            if (map.containsKey(sum - k)) {
                count += map.get(sum - k);
            }
            map.put(sum, map.getOrDefault(sum, 0) + 1);
        }

        return count;
    }
}