Add solution #18

Author: JoshCrozier

README.md

@@ -23,6 +23,7 @@
 15|[3Sum](./0015-3sum.js)|Medium|
 16|[3Sum Closest](./0016-3sum-closest.js)|Medium|
 17|[Letter Combinations of a Phone Number](./0017-letter-combinations-of-a-phone-number.js)|Medium|
+18|[4Sum](./0018-4sum.js)|Medium|
 19|[Remove Nth Node From End of List](./0019-remove-nth-node-from-end-of-list.js)|Medium|
 20|[Valid Parentheses](./0020-valid-parentheses.js)|Easy|
 21|[Merge Two Sorted Lists](./0021-merge-two-sorted-lists.js)|Easy|

solutions/0018-4sum.js

@@ -0,0 +1,59 @@
+/**
+ * 18. 4Sum
+ * https://leetcode.com/problems/4sum/
+ * Difficulty: Medium
+ *
+ * Given an array nums of n integers, return an array of all the unique
+ * quadruplets [nums[a], nums[b], nums[c], nums[d]] such that:
+ * - 0 <= a, b, c, d < n
+ * - a, b, c, and d are distinct.
+ * - nums[a] + nums[b] + nums[c] + nums[d] == target
+ *
+ * You may return the answer in any order.
+ */
+
+/**
+ * @param {number[]} nums
+ * @param {number} target
+ * @return {number[][]}
+ */
+var fourSum = function(nums, target) {
+  const result = [];
+
+  nums.sort((a, b) => a - b);
+
+  for (let i = 0; i < nums.length - 3; i++) {
+    for (let j = i + 1; j < nums.length - 2; j++) {
+      let high = nums.length - 1;
+      let low = j + 1;
+
+      while (low < high) {
+        const sum = nums[i] + nums[j] + nums[low] + nums[high];
+
+        if (sum === target) {
+          result.push([nums[i], nums[j], nums[low], nums[high]]);
+          while (nums[low] === nums[low + 1]) {
+            low++;
+          }
+          while (nums[high] === nums[high - 1]) {
+            high--;
+          }
+          low++;
+          high--;
+        } else if (sum < target) {
+          low++;
+        } else {
+          high--;
+        }
+      }
+      while (nums[j] === nums[j + 1]) {
+        j++;
+      }
+    }
+    while (nums[i] === nums[i + 1]) {
+      i++;
+    }
+  }
+
+  return result;
+};