LeetCode - Subarray Sum Equals K

Question Definition

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].

   My Java Solution

   public int subarraySum(int[] nums, int k) {
    int[] sum = new int[nums.length + 1];
    for(int i = 1; i <= nums.length; i++)
        sum[i] = sum[i - 1] + nums[i - 1];
    int count = 0;
    for(int i = 0; i <= nums.length; i++){
        for(int j = i + 1; j <= nums.length; j++){
            if(sum[j] - sum[i] == k) count++;
        }
    }
    return count;
   }
   

   Best Java Solution

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