560. 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

Solution： PreSum + HashMap

思路：
Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

public class Solution {
    public int subarraySum(int[] nums, int k) {
        int sum = 0, result = 0;
        Map<Integer, Integer> preSumCount = new HashMap<>();
        preSumCount.put(0, 1);
        
        for (int i = 0; i < nums.length; i++) {
            sum += nums[i];
            if (preSumCount.containsKey(sum - k)) {
                result += preSumCount.get(sum - k);
            }
            preSumCount.put(sum, preSumCount.getOrDefault(sum, 0) + 1);
        }
        
        return result;
    }
}