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
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 + HashTable

Complexity

O(N) O(N)

Code

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