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) {
+    
+};