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

work/Medium/Array/15.3sum.js

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+/*
+ * 15. 3Sum
+ * Difficulty: Medium
+ * https://leetcode.com/problems/3sum/
+ * 
+ * ──────────────────────────────────────────────────
+ * 
+ * Given an integer array nums, return all the triplets [nums[i],
+ * nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] +
+ * nums[j] + nums[k] == 0.
+ * 
+ * Notice that the solution set must not contain duplicate triplets.
+ * 
+ *  
+ * 
+ * Example 1:
+ * 
+ * Input: nums = [-1,0,1,2,-1,-4]
+ * Output: [[-1,-1,2],[-1,0,1]]
+ * Explanation: 
+ * nums[0] + nums[1] + nums[2] = (-1) + 0 + 1 = 0.
+ * nums[1] + nums[2] + nums[4] = 0 + 1 + (-1) = 0.
+ * nums[0] + nums[3] + nums[4] = (-1) + 2 + (-1) = 0.
+ * The distinct triplets are [-1,0,1] and [-1,-1,2].
+ * Notice that the order of the output and the order of the triplets
+ * does not matter.
+ * 
+ * Example 2:
+ * 
+ * Input: nums = [0,1,1]
+ * Output: []
+ * Explanation: The only possible triplet does not sum up to 0.
+ * 
+ * Example 3:
+ * 
+ * Input: nums = [0,0,0]
+ * Output: [[0,0,0]]
+ * Explanation: The only possible triplet sums up to 0.
+ * 
+ *  
+ * 
+ * Constraints:
+ * 
+ * 	• 3 <= nums.length <= 3000
+ * 
+ * 	• -10^5 <= nums[i] <= 10^5
+*/
+
+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+var threeSum = function(nums) {
+    
+};

work/Medium/Array/167.two-sum-ii-input-array-is-sorted.js

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+/*
+ * 167. Two Sum II - Input Array Is Sorted
+ * Difficulty: Medium
+ * https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/
+ * 
+ * ──────────────────────────────────────────────────
+ * 
+ * Given a 1-indexed array of integers numbers that is already sorted in
+ * non-decreasing order, find two numbers such that they add up to a
+ * specific target number. Let these two numbers be numbers[index1] and
+ * numbers[index2] where 1 <= index1 < index2 <= numbers.length.
+ * 
+ * Return the indices of the two numbers index1 and index2, each
+ * incremented by one, as an integer array [index1, index2] of length 2.
+ * 
+ * The tests are generated such that there is exactly one solution. You
+ * may not use the same element twice.
+ * 
+ * Your solution must use only constant extra space.
+ * 
+ *  
+ * 
+ * Example 1:
+ * 
+ * Input: numbers = [2,7,11,15], target = 9
+ * Output: [1,2]
+ * Explanation: The sum of 2 and 7 is 9. Therefore, index1 = 1, index2 =
+ * 2. We return [1, 2].
+ * 
+ * Example 2:
+ * 
+ * Input: numbers = [2,3,4], target = 6
+ * Output: [1,3]
+ * Explanation: The sum of 2 and 4 is 6. Therefore index1 = 1, index2 =
+ * 3. We return [1, 3].
+ * 
+ * Example 3:
+ * 
+ * Input: numbers = [-1,0], target = -1
+ * Output: [1,2]
+ * Explanation: The sum of -1 and 0 is -1. Therefore index1 = 1, index2
+ * = 2. We return [1, 2].
+ * 
+ *  
+ * 
+ * Constraints:
+ * 
+ * 	• 2 <= numbers.length <= 3 * 10^4
+ * 
+ * 	• -1000 <= numbers[i] <= 1000
+ * 
+ * 	• numbers is sorted in non-decreasing order.
+ * 
+ * 	• -1000 <= target <= 1000
+ * 
+ * 	• The tests are generated such that there is exactly one solution.
+*/
+
+/**
+ * @param {number[]} numbers
+ * @param {number} target
+ * @return {number[]}
+ */
+var twoSum = function(numbers, target) {
+    let i = 0, j = numbers.length - 1;
+
+    while (i < j) {
+        const curr = numbers[i] + numbers[j];
+        if (curr === target){
+            return [i + 1, j + 1];
+        }else{
+            if (curr < target){
+                i++;
+            }else{
+                j--;
+            }
+        }
+    }
+    return [];
+};

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

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+/*
+ * 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
+};