/*
 * 560. Subarray Sum Equals K
 * Difficulty: Medium
 * https://leetcode.com/problems/subarray-sum-equals-k/
 * 
 * ──────────────────────────────────────────────────
 * 
 * Given an array of integers nums and an integer k, return the total
 * number of subarrays whose sum equals to k.
 * 
 * A subarray is a contiguous non-empty sequence of elements within an
 * array.
 * 
 *  
 * 
 * Example 1:
 * 
 * Input: nums = [1,1,1], k = 2
 * Output: 2
 * 
 * Example 2:
 * 
 * Input: nums = [1,2,3], k = 3
 * Output: 2
 * 
 *  
 * 
 * Constraints:
 * 
 * 	• 1 <= nums.length <= 2 * 10^4
 * 
 * 	• -1000 <= nums[i] <= 1000
 * 
 * 	• -10^7 <= k <= 10^7
*/

/**
 * @param {number[]} nums
 * @param {number} k
 * @return {number}
 */
var subarraySum = function(nums, k) {
  const map = new Map();
  map.set(0, 1);

  let sum = 0;
  let count = 0;

  for (let n of nums){
    sum += n;

    if(map.has(sum - k)){
      count += map.get(sum - k);
    }

    map.set(sum, (map.get(sum) || 0) + 1);
  }
  return count
};