560 Subarray Sum Equals K

560. Subarray Sum Equals K

1. Question

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:

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].

2. Implementation

(1) HashTable

class Solution {
    public int subarraySum(int[] nums, int k) {
        Map<Integer, Integer> map = new HashMap<>();
        map.put(0, 1);
        int sum = 0;
        int res = 0;

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

3. Time & Space Complexity

时间复杂度O(n), 空间复杂度O(n)