feat(problems): add medium array problem solutions for 3Sum, Two Sum II, and Subarray Sum Equals K

work/Medium/Array/560.subarray-sum-equals-k.js

@@ -0,0 +1,59 @@
+/*
+ * 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
+};