560. Subarray Sum Equals K (https://leetcode.com/problems/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.

560. Subarray Sum Equals K (#1 Array + Brutal force).java

// Array + Brutal force solution
// Time: O(n ^ 2), 212ms
// Space: O(1), 40.6mb
class Solution {
    public int subarraySum(int[] nums, int k) {
        int ans = 0;
        
        for(int i = 0; i < nums.length; i++) {
            int sum = 0;
            for(int j = 0; i + j < nums.length; j++) {
                sum += nums[i + j];
                if(sum == k) ans++;
            }
        }
        
        return ans;
    }
}

560. Subarray Sum Equals K (#1 Array + Hash table).java

// Array + Hash table solution: store the <prefix sum, count> in a table; idea like 1 two sum
// Time: O(n), 11ms
// Space: O(n), 42.3mb
class Solution {
    public int subarraySum(int[] nums, int k) {
        int ans = 0;
        
        // Init the <sum, count> table
        Map<Integer, Integer> table = new HashMap<>();
        table.put(0, 1);
        
        int sum = 0;
        for(int i = 0; i < nums.length; i++) {
            sum += nums[i];
            
            // Look up the table for prefix with a sum of sum - k: sum(i,j) = sum(0,j) - sum(0,i)
            if(table.containsKey(sum - k)) ans += table.get(sum - k);
            
            table.put(sum, table.getOrDefault(sum, 0) + 1);
        }
        
        return ans;
    }
}